Methods and systems for using a distributed storage to its maximum bandwidth

ABSTRACT

Using a distributed storage to its maximum bandwidth including the following steps: for each group of at least one assembling device, selecting a subgroup of fractional-storage CDN servers according to at least one criterion, whereby a plurality of server subgroups are selected for a plurality of assembling device groups. And retrieving, using a pull protocol, by the assembling devices from the subgroups of servers, erasure-coded fragments associated with multiple segments of contents, until the aggregated bandwidth used for retrieving the fragments approaches the aggregated bandwidth of the servers included in the subgroups, and as long as the aggregated bandwidth used for delivering each segment does not exceed the aggregated bandwidth of the servers storing the fragments generated from the segment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/105,683, filed Oct. 15, 2008.

BACKGROUND

Distributed content delivery systems may have certain availablecommunication bandwidths, through which they transport data torecipients. It is often desirable to utilize as much of the availablebandwidths as possible. However, the higher the number of sources aclient has to obtain content fragments from, the more difficult it is toutilize the system's aggregated available bandwidth to its full extent.

BRIEF SUMMARY

In one embodiment, a method comprising: for each group of at least oneassembling device, selecting a subgroup of fractional-storage CDNservers according to at least one criterion, whereby a plurality ofserver subgroups are selected for a plurality of assembling devicegroups; and retrieving using a fragment pull protocol, by the assemblingdevices from the subgroups of servers, erasure-coded fragmentsassociated with segments of contents, until the aggregated bandwidthused for retrieving the fragments approaches the aggregated bandwidth ofthe servers included in the subgroups, and as long as the aggregatedbandwidth used for delivering each segment does not exceed theaggregated bandwidth of the servers storing the fragments encoded fromthe segment.

In one embodiment, a method comprising: obtaining by assembling devices,from subgroups of fractional-storage servers, erasure-coded fragmentsencoded from segments of at least one content; and changing at least oneof the servers in the subgroups on-the-fly from time to time utilizing afragment pull protocol; wherein different mixtures of segments can besimultaneously obtained by approximately any number of assemblingdevices, until the aggregated bandwidth used for obtaining the segmentsapproaches the available aggregated bandwidth of the servers, and aslong as the aggregated bandwidth used to deliver each segment does notexceed the aggregated bandwidth of the servers storing the fragmentsassociated with the segment.

In one embodiment, a distributed system comprising: CDN serversconfigured to store enough erasure-coded fragments to reconstructsegments of streaming content; for each segment, approximately each CDNserver is configured to store less than the minimal amount oferasure-coded fragments needed to reconstruct the segment; thedistributed system is further configured to provide erasure-codedfragments to assembling devices at a total throughput approaching itsmaximum aggregated bandwidth, while supplying erasure-coded fragments toreconstruct approximately any possible mixture of streaming contents;wherein one or more of the assembling devices can replace one or more ofthe servers they retrieve from, with different servers, approximatelyinstantaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments are herein described, by way of example only, withreference to the accompanying drawings. No attempt is made to showstructural details of the embodiments in more detail than is necessaryfor a fundamental understanding of the embodiments. In the drawings:

FIG. 1 to FIG. 3 illustrate changes in content consumption.

FIG. 4A is a flow diagram of one method in accordance with oneembodiment.

FIG. 4B is a flow diagram of one method in accordance with oneembodiment.

FIG. 5 illustrates an assembling device obtaining erasure-codedfragments from fractional-storage servers.

FIG. 6 illustrates real-time content segmentation, encoding, anddistribution.

FIG. 7 illustrates real time fragment retrieval, segment reconstruction,and content presentation.

FIG. 8 illustrates a broadcast-like effect.

FIG. 9 illustrates utilization of the entire aggregated bandwidth offractional-storage servers for multiple content delivery.

FIG. 10 illustrates one embodiment of segmenting content, encoding thesegments into erasure-coded fragments, distributing the fragments tofractional-storage servers, and obtaining the fragments by assemblingdevices and assembling servers.

FIG. 11 illustrates fast real time fragment retrieval.

FIG. 12 to FIG. 15 illustrate various embodiments of fragment pullprotocols.

FIG. 16 illustrates retrieving fragments and compensating for failures.

FIG. 17 illustrates fractional-storage servers having the same bandwidthcapability.

FIG. 18 illustrates fractional-storage servers having differentbandwidth capabilities.

FIG. 19 and FIG. 20 illustrate a case where a fractional-storage serverhas failed.

FIG. 21 illustrates a server failure due to network congestion.

FIG. 22 illustrates retrieving fragments according to locality.

FIG. 23 and FIG. 24 illustrate different embodiments of contentsegmentation.

FIG. 25 illustrates distribution and storage of erasure-coded fragmentson fractional-storage servers.

FIG. 26 illustrates three examples of changes made to redundancy factorsaccording to changes in demand.

FIG. 27 illustrates a server array managing a pool of bandwidthamplification devices.

FIG. 28 illustrates a fractional-storage server controlling twobandwidth amplification devices.

FIG. 29 and FIG. 30 illustrate dynamic bandwidth amplification byallocating bandwidth amplification devices according to content demand.

FIG. 31 illustrates bandwidth amplification using duplicatederasure-coded fragments.

FIG. 32 illustrates bandwidth amplification using unique erasure-codedfragments.

FIG. 33 illustrates a hybrid Servers-P2P system.

FIG. 34 illustrates real-time content segmentation, encoding, anddistribution.

FIG. 35 illustrates boosting fractional-storage servers' bandwidth usingP2P devices.

FIG. 36 illustrates operation of hybrid pull and push protocols.

FIG. 37 illustrates fractional-storage servers located on the Internetbackbone.

FIG. 38 illustrates an assembling server located at a network juncture.

DETAILED DESCRIPTION

In one embodiment, a storage system includes fractional-storage serversstoring erasure-coded fragments. The erasure-coded fragments areassociated with segments of contents, which may be streaming content.The storage system is approximately insensitive to the mixture of theconsumed contents as long as the aggregated throughput is below thetotal throughput of the fractional-storage servers. The contents areconsumed by assembling devices that obtain the erasure-coded fragmentsfrom the fractional-storage servers and decode the obtainederasure-coded fragments to reconstruct the segments. FIG. 1 illustratesone example of a server array, including N fractional-storage servers(399 a to 399(N)), and storing content A, which includes erasure-codedfragments 310 a to 310(N), and content B, which includes erasure-codedfragments 320 a to 320(N). Each server is connected to the network 300with a fragment delivery bandwidth capability B 339. Therefore, the Nservers have an aggregated bandwidth of B×N. A first group of assemblingdevices 329 a consumes content A at an average bandwidth Ba 349 a. Asecond group of assembling devices 329 b consumes content B at anaverage bandwidth Bb 349 b. Since all of the servers participate in thetransmission of the two contents, the first and second groups canpotentially consume all server bandwidth, up to the limit whereBa+Bb=N×B, with any ratio of demand between the first and secondcontents, and with no special provisions to be made when storing theerasure-coded fragments related to the two contents in thefractional-storage server array.

FIG. 2 illustrates the case where the first group 328 a, which consumescontent A, becomes larger than 329 a, with a larger bandwidth Ba 348 a.The second group 328 b, which consumes content B, becomes smaller than329 b, with a smaller bandwidth Bb 348 b, such that Ba is about the sameas Bb. In this case, the array can still be exploited up to theaggregated bandwidth, since, as before, Ba+Bb can still be almost ashigh as N×B. FIG. 3 illustrates the case where the first group hasdisappeared, allowing the second group 327 b, which consumes content B,to extract an aggregated bandwidth of Bb 347 b that can potentiallyreach the limits of the server array, such that Bb=N×B. Again, this isachieved without updating the erasure-coded fragments associated withcontent A and content B, and without using inter-server interaction.

In some embodiments, the ability to utilize the aggregated bandwidth ofapproximately all of the participating servers, for the delivery ofabout any mixture of contents with about any mixture of contentbandwidth demand, is made possible by one or more of the following: (i)each assembling device selecting a subgroup of the least loadedfractional-storage servers from which to retrieve the necessary numberof erasure-coded fragments to reconstruct a segment or several segments(least-loaded server selection criterion); or (ii) each assemblingdevice approximately randomly selecting a subgroup from which toreconstruct a segment or several segments, such that when manyassembling devices select at random, the various fractional-storageservers are selected approximately the same number of times (or inproportion to their available resources, such as unutilized bandwidth),which in turn balances the load between the participating servers(random server selection criterion). It is noted that (i) the selectionsmay be made by either the assembling devices themselves, or may be madefor the assembling devices by a control server, which then communicatesthe selections to each of the assembling devices; (ii) the selectionsmay be made approximately for each segment, or for a group of segments,or only once per content at the beginning of the content; (iii) someassembling devices may use an approximately random server selectioncriterion, while other assembling devices may use least-loaded serverselection criterion; (iv) the least-loaded selected servers may beselected out of a portion of all available fractional-storage servers.For example, the least-loaded servers may be selected fromfractional-storage servers with low latency response or with low hopcount to the assembling device; (v) the least-loaded servers may includeservers having the most unutilized bandwidth. Additionally oralternatively, it may include servers having any unutilized bandwidthleft to serve additional assembling devices; (vi) an approximatelyrandom or least-loaded selection of servers may be made such that allservers are selected to determine a subgroup, or it can be made suchthat every time selections are made, only some servers are selected,while the others remain as before. In these cases, the assembling deviceruns a process in which only a small portion of the servers currently inthe serving subgroup are reselected. In the case of approximately randomselection, the assembling device may randomly select the number ofservers in the serving subgroup for random selection (reselection inthis case, since they are replacing other servers already in the servingsubgroup of the specific assembling device), such that eventually, overtime, all servers within the serving subgroup have the chance to berandomly reselected. In the case of least-loaded server selection, onlythe most loaded servers within the serving subgroup may be selected andreplaced by less-loaded servers. In one embodiment, reselection ofsubgroups is done while retrieving the fragments associated with thebeginnings of the contents. Given a large number of assembling devices,and a large number of reselections associated with beginnings ofcontents, the system gets balanced for maximum bandwidth utilization.

FIG. 4A is a flow diagram illustrating one method, comprising thefollowing steps: In optional step 7260, distributing the fragments tothe servers with approximately no prior knowledge of the futurebandwidth demand for retrieving each content. In step 7261, for eachgroup of at least one assembling device, selecting a subgroup offractional-storage CDN servers according to at least one criterion,whereby a plurality of server subgroups are selected for a plurality ofassembling device groups. In step 7262, retrieving using a fragment pullprotocol, by the assembling devices from the subgroups of servers,erasure-coded fragments associated with multiple segments of contents,until the aggregated bandwidth used for retrieving the fragmentsapproaches the aggregated bandwidth of the servers included in thesubgroups, and as long as the aggregated bandwidth used for deliveringeach segment does not exceed the aggregated bandwidth of the serversstoring the fragments generated from the segment. And In optional step7263, repeating the step of selecting the subgroup of serversoccasionally on-the-fly. Optionally, the fragment pull protocol is afragment pull protocol for high latency, each server stores at least onefragment per approximately each segment of each content, and whereby itis possible for any content to be retrieved utilizing a bandwidthapproaching the aggregated bandwidth of the servers. Optionally, thereselection is performed by a control server, whereby the decisionsregarding the selected servers are transmitted to the relevantassembling devices. Optionally, selecting the subgroup of serversaccording to at least one criterion comprises selecting the servershaving lower loads and lower latencies than most of their alternatives,and the fragment pull protocol is a fragment pull protocol for highlatency. Optionally, the reselection is performed by each assemblingdevice for itself, and the erasure-coding is rateless-coding potentiallyresulting in fragments having a limitless redundancy factor. Optionally,the reselection is performed by each assembling device, the reselectionis performed approximately per each segment or group of segments, andapproximately all of the assembling devices use the same criterion forselecting the subgroups. Optionally, a first portion of the assemblingdevices use a first criterion and a second portion of the assemblingdevices use a second criterion for selecting the servers in thesubgroups. Optionally, the method further comprising reselecting thesubgroups while retrieving the fragments associated with the beginningsof the contents. Optionally, the method further comprising repeating thestep of selecting the servers occasionally on-the-fly for only some ofthe servers.

FIG. 4B is a flow diagram illustrating one method, comprising thefollowing steps: In optional step 7270, distributing the fragments tothe servers with approximately no prior knowledge of the futurebandwidth demand for obtaining each segment. In step 7271, obtaining byassembling devices, from subgroups of fractional-storage servers,erasure-coded fragments associated with multiple segments of at leastone content, wherein different mixtures of segments can besimultaneously obtained by approximately any number of assemblingdevices, until the aggregated bandwidth used for obtaining the segmentsapproaches the available aggregated bandwidth of the servers, and aslong as the aggregated bandwidth used to deliver each segment does notexceed the aggregated bandwidth of the servers storing the fragmentsassociated with the segment. And In step 7272, changing at least one ofthe servers in the subgroups on-the-fly from time to time utilizing afragment pull protocol. Optionally, each server stores at least onefragment per approximately each segment of each content, and theassembling devices are using a fragment pull protocol for obtaining atleast most of the fragments from the servers. Optionally, the assemblingdevices are using a fragment pull protocol for obtaining at least mostof the fragments from the servers, and the step of changing at least oneof the servers in the subgroups comprises reselecting the serversapproximately randomly from all of the available servers; and whereinthe erasure-coding is rateless-coding potentially resulting in fragmentshaving a limitless redundancy factor. Optionally, the assembling devicesare using a fragment pull protocol for obtaining the fragments from theservers, and the step of changing at least one of the servers in thesubgroups comprises changing approximately randomly only a portion ofthe servers in the subgroups. Optionally, the servers arefractional-storage CDN servers located close to or on the Internetbackbone, and the assembling devices obtaining the fragments via atleast two sub-transmissions transmitted by the servers.

FIG. 5 illustrates one embodiment of a server array includingfractional-storage servers 399 a to 399(N) storing erasure-codedfragments 390 a to 390(N) associated with content. In order forassembling device 661 to reconstruct a segment 101 a of the content, ithas to retrieve at least K erasure-coded fragments. In one example, k=4and the assembling device 661 chooses approximately randomly from whichservers to retrieve the 4 different erasure-coded fragments. It choosesto retrieve fragments 390 a, 390 c, 390(N−1) and 390(N), which are notedas group 573, and reconstruct the segment 101 a. Consequent segments ofthe content are reconstructed in a similar fashion, and the content mayeventually be fully retrieved by combining all relevant segments. If theassembling device 661 cannot reconstruct the segment 101 a, it retrievesone or more additional unique erasure-coded fragments, and tries again.

In one embodiment, the content being distributed supports streampresentation, and segment 101 a is of small size, to enable contentpresentation by assembling device 661 shortly after beginning thereception of the segment (or any other segment of the content). Forexample, segment 101 a is 96 KByte, allowing a 5 Mbps download speedreceiver to obtain the entire segment (by requesting enougherasure-coded fragments to enable the reconstruction of the segment, andsuch that the total size received of all requested erasure-codedfragments is slightly larger than the segment) after approximately 0.2seconds from request, and beginning the presentation shortly or rightafter the successful decoding and reconstruction of segment 101 a.

In some embodiments, the fragments are small enough to be contained inone packet. In one embodiment, each fragment is about 1400 bytes, andcan fit into one UDP or RTP packet transmitted over Ethernet. Thestateless nature of UDP and RTP allows the servers to send one packetwith one fragment very quickly, without the need for any acknowledgementor hand shaking. In some embodiments, the fragment pull protocolrequests use one stateless packet, like UDP or RTP. In one embodiment,the assembling device requests about 100 fragments approximately inparallel, using 100 separate requests or one or few aggregated requests.About 100 servers respond by sending about 100 fragments, eachencapsulated in one stateless packet, after a short delay, and theassembling device receives the fragments within a fraction of a second.Assuming an Internet round trip delay of 100 ms, and server processinglatency of 100 ms, then after 200 ms the assembling device startsreceiving all 100 fragments. With a modem of 5 Mbps, and assuming 1400bytes per fragment, all 100 fragments are received 1400×100×8/5 Mbps=224ms after the initial delay, meaning that content can be presented200+224=424 ms after request (decoding and other process time has beenignored in this example).

The following embodiments describe processes for on-the-flyerasure-coded fragment retrieval from fractional-storage servers.

In one embodiment, a method for obtaining erasure-coded fragments fromfractional-storage servers to reconstruct a segment includes thefollowing steps: (i) identifying the next segment to be obtained;optionally, the segments are approximately sequential segments ofstreaming content obtained according to their sequential order; (ii)optionally, determining the minimum number of fragments needed toreconstruct the segment; (iii) are enough identified relevant servers(i.e. servers storing the required fragments) available from the processof obtaining prior segment/s? (iv) if no, identifying enough relevantservers; (v) if yes, requesting enough fragments from the identifiedrelevant servers; if less than enough fragments are obtained from theidentified relevant servers, go back to step iv and identify additionalrelevant server/s; (vi) reconstruct the segment from the obtainedfragments; and (vii) optionally, go back to step i to obtain the nextsegment.

In one embodiment, a method for obtaining erasure-coded fragments fromfractional-storage servers to reconstruct multiple segments includes thefollowing steps: (i) identifying multiple segments to be obtained,optionally according to their sequential order; (ii) optionally,determining the minimum number of fragments needed to reconstruct thesegment; (iii) optionally, determining the number of fragments to beobtained approximately in parallel; (iv) are enough identified relevantservers available from the process of obtaining prior segment/s? (v) ifno, identifying enough relevant servers; (vi) if yes, requesting enoughfragments from the identified relevant servers, optionally in paralleland according to the sequential order of the segments; (vii) if lessthan enough fragments are obtained from the identified relevant servers,go back to step iv and identify additional relevant server/s; (viii)reconstructing the segment/s from the obtained fragments; and (ix)optionally, go back to step i to obtain the next segments.

In one embodiment, a method for obtaining erasure-coded fragments fromfractional-storage servers to reconstruct a segment in a burst modeincludes the following steps: (i) identifying the next segment to beobtained; (ii) optionally, determining the minimum number of fragmentsneeded to reconstruct the segment; (iii) are more than the minimumnumber of relevant servers available from the process of obtaining priorsegment/s? (iv) if no, identifying more than the minimum relevantservers; (v) if yes, requesting more than the minimum number offragments needed to reconstruct the segment; if less than enoughfragments are obtained, go back to step iv and identify additionalrelevant server/s; (vi) reconstructing the segment from the obtainedfragments; and (vii) optionally, go back to step i to obtain the nextsegment.

The various methods for obtaining erasure-coded fragments from thefractional-storage servers for reconstructing one or more segments maybe combined as needed. In one example, the initial segment/s areobtained using a burst mode and the following segments are retrievedwithout requesting extra fragments. In another example, the initialsegment/s are obtained approximately in parallel and optionally using aburst mode, and the following segments are obtained one by one andoptionally without requesting extra fragments. The fragments may beobtained using a pull protocol and/or a push protocol. Moreover, theservers from which to retrieve the fragments may be selected accordingto one or more of the various discussed methods for selecting theservers and/or load balancing the servers.

In some embodiments, a push protocol is used to obtain fragments. A pushprotocol may be implemented using one transmission carrying fragmentsfrom a source server to a destination receiver, or may be implementedusing a plurality of sub-transmissions. When using sub-transmissions,each sub-transmission transports a fraction of the fragments needed forsegment reconstruction. Segments may be reconstructed from fragmentsreceived via sub-transmissions after obtaining decodable sets oferasure-coded fragments; optionally one set per segment. Asub-transmission may be transported using an IP stream such as RTP, anHTTPS session, or any other protocol suitable for transporting asequence of fragments between a source server and a destinationassembling device.

FIG. 5 illustrates one embodiment, in which content is segmented anderasure-coded. Fragments 390 a to 390(N), belonging to a first segment,are distributed to servers 399 a to 399(N) respectively. Other fragmentsbelonging to subsequent segments are similarly distributed to servers399 a to 399(N). The servers may use a push protocol to transport thefragments to an assembling device. A push protocol sub-transmission maycomprise a sequence of fragments associated with multiple segments. Inone example, the fragments are ordered according to the sequential orderof the segments in a streaming content. Server 399 a sends a firstsub-transmission to a destination assembling-device. Optionally, thefirst sub-transmission comprises a sequence of fragments starting withfragment 390 a, associated with the first segment, and continuing withfragments belonging to subsequent segments. Server 399 c sends a secondsub-transmission to the destination assembling-device, optionallystarting with fragment 390 c, associated with the first segment, andcontinuing with fragments belonging to subsequent segments. In a similarfashion, servers 399(N−1) and 399(N) send additional sub-transmissionsto the destination assembling-device, each comprising a unique fragmentsequence.

When using a push transmission, the assembling device does notexplicitly ask for each fragment, but instead instructs each of thedifferent servers to start sending it a fragment sequence using asub-transmission. The destination assembling-device receives thesub-transmissions sent by servers 399 a, 399 c, 399(N−1) and 399(N). Itgathers 573 the first fragment from each sub-transmission to reconstructthe first segment 101 a. In a similar fashion, additional fragmentsbelonging to subsequent segments are obtained from thesub-transmissions, and used to reconstruct the segments. It is notedthat any combination of sub-transmissions may be used, as long as adecodable set of fragments is obtained per each segment. It is alsonoted that FIG. 5 illustrates a non-limiting embodiment and asub-transmission may include two or more unique erasure-coded fragmentsper segment.

In one embodiment, the push sub-transmissions is synchronous (allservers sending the fragments of each segment at approximately the sametime). In another embodiment, the push sub-transmission is asynchronousand the arrival of different fragments associated with a specificsegment at the assembling device side may be spread over a long period.This may occur, as an example, when some push servers are faster thanothers. In one embodiment using asynchronous sub-transmissions, theassembling device aggregates whatever fragments it can beforepresentation time of each segment, and then optionally supplementsfragments using a pull retrieval process. A server that does not sendfragments fast enough, and therefore usually causes supplementalrequests, may be ordered to stop the sub-transmission. Another servermay be requested, optionally by the assembling device, to replace theslow server by initiating a new sub-transmission.

In one embodiment, the push-transmissions carry more erasure-codedfragments than needed for segment reconstruction. In one embodiment, thepush transmissions carry fewer erasure-coded fragments than needed forsegment reconstruction, and the remaining fragments are pulled by theassembling device.

The term “erasure coding” as used herein denotes a process in which asequence of erasure-coded fragments can be generated from a segment suchthat the segment can be reconstructed from any or almost any subset ofthe erasure-coded fragments of size equal to or somewhat larger than thesize of the segment (sometimes may be referred to as “enougherasure-coded fragments” or “sufficient subset of fragments”). Examplesof erasure codes include, but are not limited to, rateless codes,Reed-Solomon codes, Tornado codes, Viterbi codes, Turbo codes, any Blockcodes, any Convolutional codes, and any other codes that are usuallyused for forward error correction (FEC).

The term “rateless coding” as used herein denotes a type of erasurecoding in which a very long, potentially limitless, sequence ofrateless-coded fragments can be generated from a segment such that thesegment can be reconstructed from any or almost any subset of therateless-coded fragments of size equal to or somewhat larger than thesize of the segment (sometimes may be referred to as “enoughrateless-coded fragments”). Examples of rateless codes include, but arenot limited to, Raptor codes, LT codes, online codes, any Fountaincodes, and any other Rateless codes.

The term “erasure-coded fragment” denotes a fragment comprising dataencoded with an erasure code (which may also be a rateless code in someembodiments). The term “rateless-coded fragment” denotes a fragmentcomprising data encoded with a rateless code.

The term “assembling device” as used herein denotes a computing devicethat retrieves erasure-coded fragments from servers over a network. Theassembling device may perform one or more of the following: (i) Decodethe retrieved erasure-coded fragments into segments. (ii) Present thecontent reconstructed from the retrieved erasure-coded fragments. (iii)Act as a bandwidth amplification device, by receiving, storing, andforwarding erasure-coded fragments. In some embodiments, the assemblingdevice may be any device located at the user premises, like an STB, PC,gaming console, DVD player, PVR device, or any other device able toretrieve erasure-coded fragments from a communication network. In someembodiments, the assembling device may be an assembling server. In someembodiments, the assembling device may be any computational device withaccess to a communication network, located at a central office, datacenter, BRAS location, ISP premises, or any other place with directnetwork connectivity. In one embodiment, the assembling device iscoupled to a display device used for content presentation.

The abbreviation CDN denotes “Content Delivery Network”. The term “CDNserver” as used herein denotes a server having one or more of thefollowing characteristics: (i) A bandwidth (CDN_BW) that is much greaterthan the average bandwidth consumed by a user premises device (User_BW)receiving video streaming content. In some examples, the CDN_BW is atleast 10 times, 100 times, 1,000 times, or 10,000 times greater than theUser_BW. (ii) The server is located outside the last mile communicationinfrastructure of the end users, such that the CDN server and the endusers are located in different networks. For example, the CDN server isnot located under a BRAS, while the end users are located under a BRAS.Moreover, in some embodiments, the CDN servers are deployed over a widearea across the Internet and optionally located close to or on theInternet backbone. In some embodiments, the CDN server does not usuallyretrieve and play streaming content. In some embodiments, the CDN serverhas a much greater storage space than the storage space of an averageplayer of streaming content.

The term “fractional-storage server” in the context of erasure-codedfragments (also applicable to “fractional-storage CDN server”), as usedherein denotes a server that (i) stores less than the minimum quantityof erasure-coded fragments required to decode the erasure-codedfragments, and (ii) where at least a meaningful quantity of the storederasure-coded fragments is not stored in order to be consumed by thefractional-storage server.

The term “streaming content” as used herein denotes any type of contentthat can begin playing as it is being delivered. Streaming content maybe delivered using a streaming protocol, a progressive downloadprotocol, or any other protocol enabling a client to begin playing thecontent as it is being delivered. Moreover, the term “streamingprotocol” includes “progressive download protocol”. In addition, theverb “streaming” refers to using a streaming protocol, using aprogressive download protocol, or using any other protocol enabling thereceiver to begin playing the content as it is being delivered.

In some embodiments, expressions like “approximately sequentialsegments” may denote one or more of the following non-limiting options:segments that are sequential (in time or according to a file's order),segments that are approximately sequential (such as segments with someinterlace, or segments without a great amount of non-sequential data),segments generated sequentially and/or approximately sequentially fromdifferent components of content (such as storing the i-frames andp-frames of a compressed content in different segments), and/or othersequential or approximately sequential segmentation after classificationor separation into different components and/or elements.

The term “redundancy factor” as used herein denotes the following ratio:(total size of the unique erasure-coded fragments generated from asegment and actually stored on the servers)/(size of the segment).

Assuming all segments have approximately the same size and all fragmentsgenerated from the segments have approximately the same size (withoutlimiting any of the embodiments), the term “storage gain” as used hereindenotes the following ratio: (size of a segment)/(size of anerasure-coded fragment). If the server stores more than oneerasure-coded fragment per segment, the storage gain denotes thefollowing ratio: (size of segment)/((size of erasure-codedfragment)*(number of stored erasure-coded fragments per segment)).

The term “approximately random” as used herein refers to, but is notlimited to, random, pseudo random, and/or based on a long list ofnumbers featuring very low autocorrelation and very low correlation withother similar lists of numbers.

In some embodiments, a broadcast-like effect is achieved by distributingto and retrieving from fractional-storage servers a broadcastchannel/live content in real time, using a combination of real timedistribution and real time retrieval techniques. In a broadcast-likeeffect, a given channel or content for broadcasting is distributed to atleast one assembling device, optionally by means of pushing relevantfragments to the assembling device, or by pulling the relevant fragmentsby the assembling device, and potentially to many assembling devices atapproximately the same time, which creates a similar effect totraditional broadcasting.

FIG. 6 illustrates one embodiment of processing a content source 700 forreal time presentation. Content examples include, but are not limitedto, a live video broadcast event, a pre-recorded show, or any real timeconditioned source. The content 700 is available at time T1=0. Thecontent 700 is segmented in real time into multiple segments, such thatthe first segment 710 a is available at T3. T3 is determined by the sizeof the segment and the selected compression scheme. For example, if anH.264 compression is used to generate an average stream of 1 Mbps, andthe size of the segment is 96 Kbytes, then T3 minus T2 equals 96KByte×8(Bits/Byte)/1 Mbps=0.77 seconds on average, where T2 is theprocess delay. If T2 is about 0.2 second, then the first segment 710 acan be ready for the next step after about 1 second from the time thatcontent 700 is first made available. Subsequent segments 710 b to 710Jare made available sequentially in time.

Next, at T4, erasure-coded fragments 720 a to 720(N) are being encodedfrom segment 710 a. At T6, the encoding process is performed for segment710 a, and all the erasure-coded fragments 720 a to 720(N) are madeavailable. In one example, the time between T4 and T6 is equal to orless than the average segment creation time, in order to allow theprocess to maintain real time performance, such that at any point intime during the on-going availability of segments, the encoding processgenerates all erasure-coded fragments without picking up any delay aboveT6 minus T3 (which is the latency between segment availability anderasure-coded fragment availability). T6 minus T1 may be typically 2-3seconds if T3 minus T2 is 0.77 seconds. T4 minus T3 may be typically afraction of a second. Similarly, erasure-coded fragments 730 a to 730(N)are being encoded from segment 710 b, and are made available at time T9.The process of fragment encoding is repeated in real time up to the lastsegment 710J of content 700.

Next, at T5 (which can potentially occur before T6, but also after T6)the erasure-coded fragments 720 a to 720(N) are distributed 740 a to aserver array. The distribution process 740 a ends at T8. In one example,T8 minus T5 is equal to or less than the average segment creation time,in order not to have delays. The process of distributing theerasure-coded fragment is repeated 740 b for erasure-coded fragments 730a to 730(N), and for all subsequent erasure-coded fragments associatedwith the next segments.

Optionally, at T7, the erasure-coded fragments 720 a to 720(N) aredistributed 750 a from the servers to groups of bandwidth amplificationdevices. In one example, the distribution ends at T10, such that T10minus T7 is equal to or less than the average segment creation time, inorder not to have delays. Subsequent erasure-coded fragments associatedwith the next segment are distributed 750 b, and the process continuesuntil the erasure-coded fragments associated with the last segment 710Jare distributed.

FIG. 7 illustrates one embodiment of real time streaming contentretrieval from fractional-storage servers. An assembling device begins aprocess of obtaining streaming content 700 for presentation. Starting atT1, the assembling device requests erasure-coded fragments 720 a to720(K). By T2, all K erasure-coded fragments are obtained, and at timeT2 b until T4, erasure-coded fragments 720 a to 720(K) are decoded intosegment 710 a. The retrieval time of the erasure-coded fragments and thesegment decoding time should be equal to or faster than thecorresponding presentation time, in order to enable a continuouspresentation, once presentation begins at T5. T2 b minus T2 is a shortdelay, and can be fractions of a second. Subsequent erasure-codedfragments 730 a to 730(K) are retrieved between T2 and T3, and aredecoded into subsequent segment 710 b between T4 and T6.

In one example, the streaming content 700 is encoded at 1 Mbps, and thesegment size is 96 Kbytes. The presentation of each segment takes about0.77 seconds. Retrieving fragments 720 a to 720(K) takes no more than0.77 seconds, meaning that the assembling device's connection bandwidthmust be 1 Mbps or higher. Decoding segment 710 a takes no more than 0.77seconds. If a small delay of 0.2 seconds is assumed for both T2 b minusT2 and T5 minus T4, then T5 can start at 0.77+0.2+0.77+0.2=1.94 secondsafter T1, meaning that presentation can begin about 2 seconds followingrequest of the first erasure-coded fragment. In another example, theretrieval process and the decoding process are performed faster than thereal time presentation bounds, therefore enabling a shorter time to playand a download rate that exceeds the presentation rate.

In one embodiment, the erasure-coded fragments 720 a to 720(K) areretrieved in approximately random order, or any other order, as long asat least the K erasure-coded fragments needed for decoding the segment710 a are available until time T2.

In one embodiment, the fragments associated with sequential segments ofstreaming content are delivered to an assembling device as a pluralityof sub-transmissions. In this case, each fractional-storage serverparticipating in the delivery of the fragments to the assembling devicesends a transmission to the assembling device comprising a sequence oferasure-coded fragments. This transmission is referred to as asub-transmission. In one example, each sub-transmission contains atleast one fragment per each sequential segment of the streaming content.In one example, the sub-transmission starts at a segment indicated bythe assembling device, and continues from that point onwards,approximately according to the sequential order of segments, until theassembling device instructs the server to stop, or until reaching thelast segment of the content. Each sub-transmission carries only afraction of the fragments (per segment) needed to reconstruct thesegments of the streaming content, such that the combination of at leasttwo sub-transmissions received by the assembling device from the serversallows the assembling device to obtain enough fragments needed toreconstruct each segment.

In one embodiment, each sub-transmission is delivered to the assemblingdevice via a streaming session, such as an RTP session, wherein the RTPpackets transport the fragment sequence approximately according to theorder of the sequential segments. In one embodiment, eachsub-transmission is delivered to the assembling device via an HTTPconnection, or other closed-loop data transfer mechanisms over TCP/IP.In one embodiment, the assembling device may change one or moretransmitting servers on the fly, by instructing the server(s) to stopsending an already active sub-transmission—as may be needed in a case ofan RTP session, and initiating new sub-transmissions from other serversinstead. Replacement of transmitting servers on the fly may be needed ina case of a server failure, network failure, or high load or latencyconditions.

FIG. 8 illustrates one example of creating a broadcast-like effect (i.e.retrieving the content while it is distributed). Streaming content 700a, which may be ready in advance or received on-the-fly, is to bereceived and presented by multiple assembling devices at approximatelythe same time. Content 700 a is segmented into segments on-the-fly, suchthat the first segment 710 a is ready shortly after the data isavailable, and subsequent segment 710 b is ready right after that.Segments 710 a and 710 b are sequentially encoded into erasure-codedfragments 782 a and 782 b correspondingly, such that the average rate ofencoding segments into erasure-coded fragments does not fall below theaverage rate of introducing new segments (as content 700 a is beingreceived for broadcast).

As the erasure-coded fragments 782 a are ready, they are distributed 783a to the fractional-storage servers. Subsequent erasure-coded fragments782 b are similarly distributed 783 b to the servers, such that theaverage rate of distributing the erasure-coded fragments associated witheach segment does not fall below the rate of introducing new segments(or in other words, such that there is approximately no piling-up ofundistributed segments). Optionally, the erasure-coded fragments 782 aare also distributed 784 a by the servers to bandwidth amplificationdevices at an average distribution rate per segment that does not fallbelow the average rate of introducing new segments.

The assembling devices obtain erasure-coded fragments 785 a associatedwith segment 710 a from the fractional-storage servers, and optionallyalso from the bandwidth amplification devices. Subsequent erasure-codedfragments, such as 785 b associated with segment 710 b, are obtained atan average rate that does not fall below the average rate of introducingthe new segments. The segment 710 a is then reconstructed from theobtained erasure-coded fragments 785 a. The subsequent segment 710 b isreconstructed from the obtained erasure-coded fragments 785 b, such thatreconstructing each segment is performed at an average rate that doesnot fall below the average rate of introducing the new segments.

Then, the reconstructed segments are presented, optionally on-the-fly,as reconstructed content 700 b. In one embodiment, the entire processend-to-end is performed in real time, such that the presentation of 700b starts at T2 minus T1 after the availability of content 700 a, andsuch that the delay of T2 minus T1 (between the availability of newsegments and their subsequent presentation by the assembling device) iskept approximately constant throughout the entire presentation of thestreaming content 700 b, once begun.

In one example, the content 700 a is a 4 Mbps video stream, and thesegment size is 96 Kbytes, meaning that new segments 710 a, 710 b aremade available at a rate of one every 0.19 seconds. Assuming that eachprocess as described takes 0.19 seconds, and that all processes areperformed sequentially (with no overlapping in time, which may bepossible for some of the processes), then the accumulated process time,which includes 710 a, 782 a, 783 a, 784 a, 785 a and 710 a, takes about6×0.19=1.14 seconds. This means that an assembling device may begin withcontent presentation 1.14 seconds after the content is first madeavailable to the system.

Still referring to FIG. 8, in one embodiment, the fragments are obtainedfrom the servers using multiple sub-transmissions, such that eachtransmitting server sends a fraction of the needed fragments to theassembling device, according to the sequential order of segments. Eachsub-transmission transmits the fragments approximately at a rate atwhich the fragments are being created on-the-fly from segments of thecontent to be received by the assembling device. According to anotherembodiment, the fragments are obtained from the servers using fragmentrequests made by the assembling device using a fragment pull protocol.

The Audio/Video compression utilized in creating content 700 a is notnecessarily a fixed rate compression, meaning that the various resultingsegments do not necessarily contain the same amount of presentationtime.

In one embodiment, once starting to retrieve a broadcast-like stream,the assembling device may use one of the following methods tosynchronize the retrieval of the stream's segments with the ongoingavailability of new segments of the stream: (i) The assembling deviceretrieves additional segments such that the average rate of obtainingnew frames approximately equals the average rate of presenting frames.(ii) The assembling device retrieves additional segments such that itdoes not try to retrieve segments that are not yet indicated as beingavailable. And (iii) The assembling device retrieves additional segmentsso as to approximately maintain a constant distance (in segments)between the most currently available segment and the segment currentlybeing retrieved.

In one embodiment, the assembling device presents the broadcast-likestream at approximately the same frame rate as the rate of producing newframes for the broadcast-like stream. In one example, the frame rate isconstant throughout the stream, such as the case of fixed 24, 25, 50, or60 frames per second.

In one embodiment, the assembling device obtains an indication regardingthe most newly available segment (per specific broadcast-like stream)for retrieval. The assembling device then starts to retrieve from themost newly available segment. In one example, the most newly availablesegment is the last segment that was distributed to thefractional-storage servers. In another example, the most newly availablesegment is a segment that was recently distributed to thefractional-storage servers, but wherein there are newer distributedsegments, which are not yet indicated as being available.

In one embodiment, the broadcast-like stream is of a pre-recordedcontent, such that it is possible to distribute the entire content tothe fractional-storage servers, and after any period of time allow thereal time consumption of the content by any number of assemblingdevices. In such a case, an indication is made to the assembling devicesregarding the real time allowance to retrieve the related segments. Theallowance can start at a certain point in time (which corresponds to thebeginning of the broadcast-like “transmission”) for the first segment,and then the allowance may continue for subsequent segments, at a ratethat approximately corresponds to sustaining the frame rate of thebroadcast-like stream.

In one embodiment, the system amplification factor (Bout/Bin) mayapproach the ratio (Tout/(R*Bstream)), where Bout is the total averageoutput bandwidth at which the fragments are obtained by the assemblingdevices, Bin is the total average bandwidth needed to deliver thefragments for storage on the servers, Tout is the total aggregatedoutgoing bandwidth of all servers, R is the redundancy factor, andBstream is the average bandwidth of the streaming content.

FIG. 9 illustrates one embodiment of using the entire aggregatedbandwidth of the fractional-storage servers for delivering multiplecontents. Approximately any number of contents having any mixture ofbandwidth demand per content may be delivered, as long as the aggregatedbandwidth demand does not exceed the aggregated bandwidth of thefractional-storage servers. In one example, broadcast-like streams 3101,3102, and 3103 are delivered to multiple assembling devices via multiplefractional-storage servers. Each stream is a live TV channel carryingmultiple TV programs. For example, stream 3101 comprises TV programs3110 to 3112, each spanning a specific time interval. The other streamscomprise of multiple TV programs as well. Before time T1, stream 3130has a bandwidth demand of 3130′ (meaning that all assembling devicesthat are currently retrieving stream 3130′ use a total bandwidth of3130′ out of the fractional-storage servers). The other streams 3120 and3110 have bandwidth demands of 3120′ and 3110′ respectively. The totalbandwidth demand of the three streams 3130′+3120′+3110′ does not exceedthe aggregated bandwidth of the fractional-storage servers 3150, andtherefore all streams are fully delivered to the assembling devices. Theload of the three streams is spread approximately equally among theparticipating fractional-storage servers, optionally because of amechanism that selects the least-loaded servers to serve each assemblingdevice, and/or a mechanism that approximately randomly selects serversto serve each assembling device. At time T1, TV program 3120 ends, andTV program 3121 starts. Program 3121′s demand 3121′ is higher than theprevious demand 3120′, and therefore a higher aggregated bandwidth isdrawn from the fractional-storage servers. Still, the aggregatedbandwidth demand of all three streams (3130′+3121′+3110′) is lower thanthe maximum possible 3150, and therefore the newly added bandwidthdemand is fully supported by the servers. Optionally, the additionaldemand created by TV program 3121 (3121′ minus 3120′) is caused by theaddition of new assembling devices that join stream 3102 and retrievingadditional erasure-coded fragments. Additionally or alternatively, theadditional demand created by TV program 3121 is caused by a higherbandwidth demand of TV program 3121, such as 3D data or higherresolution. Newly added assembling devices may choose fractional-storageservers from which to retrieve, according to a least-loaded serverselection criterion and/or an approximately random server selectioncriterion, and therefore the total load is still spread approximatelyequally among the participating servers. At time T2, TV program 3110ends, and a new program 3111 begins, which is less popular, andtherefore creates a lower bandwidth demand 3111′. The result is adecrease in the total delivered bandwidth. At time T3 TV program 3130ends, and TV program 3131 starts with a higher bandwidth demand of3131′. At time T4 both TV programs 3111 and 3121 end, and two newprograms 3112 and 3122 start. TV program 3112 is highly popular andtherefore generates a large bandwidth demand 3112′. Program 3122 is notpopular, and therefore generates a limited bandwidth demand 3122′. Someof the additional bandwidth needed by program 3112 is taken from serversthat stop serving assembling devices previously retrieving program 3121,such that the aggregated bandwidth of all three streams(3131′+3122′+3112′) is still below the maximum possible bandwidth 3150,despite the fact that program 3112 is generating a large bandwidthdemand. This example illustrates how the fractional-storage serverssupport almost any demand mixture, as long as the aggregated demand ofall streams is kept below the aggregated maximum capacity of the servers3150. Consequently, the distribution of all of the streams to thefractional-storage servers is approximately unrelated to the changes inbandwidth demand for programs carried by each stream; each stream can beregarded as a sequence that is segmented, erasure-encoded, anddistributed to the participating servers. There is no need to accountfor demand variations during the distribution of each stream, nor isthere a need to know in advance the bandwidth demand for each stream orfor each program within each stream. It is noted that the demandvariations are illustrated as instant variations, but may also begradual and may occur during a program and not necessarily when oneprogram ends and the other begins.

FIG. 10 illustrates one embodiment of a fractional-storage systemstoring erasure-coded fragments. Content 100, which may optionally bestreaming content, is segmented into content segments 101 a, 101 b to101 j (for brevity referred to as segments). Each of the segments isencoded into erasure-coded fragments. For example, segment 101 a isencoded into erasure-coded fragments 390 a to 390(N). The erasure-codedfragments are distributed to the fractional-storage servers 399 a to399(N) and/or to the bandwidth amplification devices 610 aa. Theerasure-coded fragments are then obtained by assembling devices like 661or proxy servers like proxy server 661 s from the fractional-storageservers 399 a to 399(N) and/or the bandwidth amplification devices 610aa. The obtained erasure-coded fragments are decoded to reconstruct thesegments. The proxy server 661 s may broadcast/multicast and/orre-stream the reconstructed content, optionally using standard streamingtechnique, to its client(s) 661 o, optionally over network 300 n. Insome embodiments, the content distribution is performed in real time. Insome embodiments, the content assembly is performed in real time and thepresentation starts a short time after the content request.

Similarly to content 100, additional contents are segmented, encodedinto erasure-coded fragments, and distributed to the fractional-storageservers and/or to the bandwidth amplification devices. Each segment maybe reconstructed independently of other segments by obtaining anddecoding enough erasure-coded fragments generated from that segment.

In some embodiments, the encoding scheme is erasure codes and/orrateless codes. In some embodiments, the fractional-storage servers 399a to 399(N) are Content Delivery Network (CDN) servers, optionallyaccessed over the public Internet. In some embodiments, the control,management, content reception, content segmentation, segment encoding,erasure-coded fragment distribution, allocation of bandwidthamplification devices, and/or other kind of central supervision andoperation may be performed by managing server(s) 393, which may be apart of the CDN network. It is noted that the term “fractional-storageserver” is not limited to a large server and, according to the context,may include a fractional-storage bandwidth amplification device, afractional-storage peer server, or other types of fractional-storageservers.

FIG. 11 illustrates one embodiment of real time streaming contentretrieval from fractional-storage servers, wherein erasure-codedfragments 720 a to 720(K) are retrieved in a fast cycle, meaning thatseveral erasure-coded fragments are obtained approximately in parallel.As a result, the interval T2 minus T1 is more or less limited only bythe download bandwidth of the assembling device's modem. Referring tothe example of FIG. 7, T2 minus T1 can be reduced from 0.77 seconds to0.15 seconds, if the modem operates at 5 Mbps (instead of 1 Mbps).

In one embodiment, T1 to T2 represents a fragment fetch cycle thatcorresponds to the beginning of streaming content to be presented (inthat case, segment 710 a is the first segment of the content, andpresentation 700 corresponds to the beginning of the streaming content),or corresponds to a certain point within the streaming content to bepresented starting this point onwards (in that case, segment 710 a is asegment within the content, and presentation 700 corresponds to playingthe content starting not from the beginning, but rather from segment 710a, located somewhere within the content). This is also known as trickplay. In one embodiment, erasure-coded fragments 720(a) to 720(K) areobtained such as to result in approximately a maximum utilization of thedownload capabilities of the assembling device, and such that the rateof requesting erasure-coded fragments results in a data arrival ratethat on average utilizes the assembling device's maximum downloadbandwidth.

FIG. 12 illustrates one embodiment of a fragment pull protocol.Assembling device 861 (also represented by protocol diagram element 810b) obtains erasure-coded fragments from fractional-storage servers 899 ato 899(N) (also represented by protocol diagram element 898), utilizingthe following steps: (i) deciding 810 a which segment to retrieve; (ii)device 861 sending requests to some of the fractional-storage serversfor erasure-coded fragments associated with the desired segment. Forexample, requests 880 a to 880(K) for erasure-coded fragments 890 a to890(K), from servers 899(a) to 899(K), correspondingly; and (iii) theservers respond by sending the requested erasure-coded fragments. Forexample, servers 899 a to 899(K) send 881 a to 881(K) erasure-codedfragments 890 a to 890(K) to device 861. The fragment request andreceipt process begins at T1 c and ends at T1 d. At time T1 d, device861 has enough erasure-coded fragments (K) to reconstruct the segmentselected at 810 a. In one embodiment, the process from T1 c to T1 doccurs in real time, in support of streaming content presentation.

FIG. 13 illustrates a similar process to FIG. 12, where request 890 bfails to result in a reception of erasure-coded fragment 890 b for anyreason (such as a server fault, network congestion, or abnormal latencyconditions). Assembling device 861 therefore issues another request882(K+1) for erasure-coded fragment 890(K+1) in response, and receives883(K+1) the additional erasure-coded fragment 890(K+1) needed toreconstruct the segment.

FIG. 14 illustrates a similar process to FIG. 12, where requests forerasure-coded fragments are loaded into one aggregated request 870, thatis sent to one of the fractional-storage servers (the receiving serveris illustrated as protocol diagram element 888 a, and will be alsoreferred to as a “relay server”). In one example, if the relay server is899(N), then, it will forward the request to additional servers 899 a to899 c (protocol element 888 b) via new requests 870 a to 870 c (onbehalf of assembling device 861). Servers 899 a to 899 c will thenrespond by sending the erasure-coded fragments 890 a to 890 c (871 a to871 c) to the assembling device 861. Server 899(N) will send 871(N)fragment 890(N) to the assembling device.

The term “fragment pull protocol for high latency” as used hereindenotes a protocol enabling an assembling device to request one or morefragments from one or more providing sources, wherein the time totransmit the one or more fragments in response to the assembling devicerequest, through the slowest communication link connecting theresponding source and the assembling device, is smaller than the roundtrip communication delay between the assembling device and theresponding source, excluding the processing time of the providingsource. For example, if the round trip communication delay betweenIsrael and the USA is about 200 ms, the assembling device requests onefragment sized about 1500 bytes, and the slowest communication link isan ADSL line connecting the assembling device at 1.5 Mbps, then the timeit takes to transmit the requested fragment through the slowestcommunication link is about 1500*8/1500000=8 ms, which is much smallerthan the round trip delay. Many of the disclosed embodiments usingfragment pull protocol may use fragment pull protocol for high latencyfor retrieving the fragments.

FIG. 15 illustrates a similar process to FIG. 12, where one or moreextra erasure-coded fragments (in addition to the needed K) arerequested in advance (illustrated as request 880(K+1) for erasure-codedfragment 890(K+1)), such that if, as an example, request 890 b fails toresult in a reception of erasure-coded fragment 890 b, assembling device861 does not have to request new erasure-coded fragments to reconstructthe segment, since there are still at least K erasure-coded fragmentsthat were successfully received and therefore the segment can bereconstructed.

In one embodiment, more fragments than needed to reconstruct a segmentare requested, such that the additional requested fragmentsapproximately compensate for fragment failure conditions. If,statistically, F fragment requests are expected not to result in thereception of a fragment (i.e. fragment loss), out of a total number ofK+F fragment requests (wherein K is the minimal number of fragmentsneeded to reconstruct a segment), then it is possible to request K+Ffragments instead of just K. In one embodiment, more than K+F fragmentsare requested, since the quantity of the received fragments is astatistical variable. In this case, K+F+S fragments are requested,wherein S is a safeguard amount of additional requests to assure that atleast K fragments are received. In one embodiment, the fragment loss Fchanges over time, and the assembling device handles the change byincreasing or decreasing the number of fragments requested per segment.In one embodiment, the assembling device may determine F based onprevious fragment failure rates.

In one embodiment, requesting K+F+S fragments for a segment will almostalways result in the reception of at least K fragments, and thereforethe assembling device may request K+F+S without being concerned aboutwhich fragment has not arrived, and without trying to activelycompensate for fragment failures by issuing additional fragmentrequests. In this case, the assembling device requests the fragments inan “open loop” fashion, meaning that it requests the K+F+S fragments,and moves on to another segment. In one embodiment, even when requestingK+F, or K+F+S fragments per segment, it is still possible not to receivethe needed K fragments. Therefore, the assembling device may compensatefor undelivered fragments by issuing additional fragment requests (a“closed loop” operation).

In one embodiment, the K+F, or K+F+S fragment requests are issuedapproximately in parallel, in order to achieve the fastest responsepossible for reconstructing a segment. In this case, the fragments startto arrive at the assembling device a short while after being requested,such that as soon as at least K out of the requested fragments arrive,the assembling device may immediately proceed with reconstructing thesegment.

In one embodiment, a method includes the steps of obtaining, by anassembling device from fractional-storage servers, erasure-codedfragments associated with a segment of streaming content; detecting afirst quantity of at least one fragment associated with the segment,which have failed to arrive at the assembling; requesting via a fragmentpull protocol for high latency approximately the first quantityfragments; and repeating the steps of detecting and requesting untilenough fragments have been obtained for reconstructing the segment.Optionally, the steps are repeated approximately sequentially on thesegments. Optionally, the streaming content comprises approximatelysequential segments. Optionally, detecting the failure comprisesdetermining which fragment has failed after not obtaining the fragmentwithin a predetermined period from issuing its request, and/or receivinga message that does not contain the actual fragment's payload. Andoptionally, the fragments are obtained via sub-transmissions transmittedby the servers.

In one embodiment, a method for retrieving erasure-coded fragmentsincludes the steps of: determining, by an assembling device, the timeremaining to reconstruct a segment of streaming content; estimating theprobability of receiving a sufficient quantity of already-orderederasure-coded fragments to reconstruct the segment during the remainingtime; and if the estimated probability is below a predefined threshold,issuing one or more additional fragment requests, using a fragment pullprotocol, until the estimated probability equals or passes thepredefined threshold. Optionally, the already-ordered fragments and theadditional fragment requests are received from fractional-storage CDNservers. Optionally, the already-ordered fragments are received via atleast two sub-transmissions transmitted by the servers. Optionally, thealready-ordered fragments are received via a fragment pull protocol.And, optionally, estimating the probability includes considering theprobability of each request to result in a fragment reception during theremaining time.

In one embodiment, an assembling device may aggregate several fragmentrequests into one message. The aggregated message is then sent to afractional-storage server, possibly in a payload of a single packet, andoptionally in order to conserve outgoing bandwidth and/or to reduce thenumber of packets needed to convey the requests. The fractional-storageserver may then read the aggregated message and act accordingly bysending a plurality of fragment responses to the assembling device. Thefragment responses may include one fragment at each payload, as is thecase of responding to a single fragment request, or it may include anaggregated response including multiple fragments at each payload.

In one embodiment, multiple segments of content, which, in one example,is streaming content, are reconstructed by an assembling deviceretrieving multiple erasure-coded fragments associated with the multiplesegments. Since a fragment request does not always result in a receptionof the fragment, some requested fragments may fail to arrive at theassembling device. Therefore, the assembling device checks (from each ofthe segments for which fragments have already been requested) whichrequested fragments have failed to result in a correct reception of afragment. For each such failure, the assembling device issues anadditional request for a fragment. The additional requests areassociated with segments for which fragments have already been requestedbefore, and therefore, in one example, the resulting fragment retrievalprocess includes the following two sub-processes: a first sub-process ofrequesting fragments associated with new segments to be reconstructed,and a second sub-process of requesting additional fragments needed tocomplement already requested fragments, in order to reconstruct thesegments. The first and second sub-processes work together, such thatthe second sub-process may complement fragments associated with a firstsegment, while the first sub-process runs ahead in an attempt to obtainfragments needed to reconstruct a second segment; wherein the secondsegment is located ahead of the first segment. The first and the secondsub-processes can also be described as two different quantities offragments being requested: a first quantity associated with the firstsub-process requests, and a second quantity associated with the secondsub-process requests.

FIG. 16 illustrates one example of retrieving fragments and compensatingthe failures. Content 100 is segmented into segments 102 a, 102 b, and102 c, and each segment is erasure-coded into four fragments, asillustrated for segment 102 a, which is coded into fragments 391 a to391 d. This example assumes that each segment can be reconstructed byobtaining any three fragments associated with it. Prior to time T1, theassembling device requests fragments 391 a, 391 b, and 391 c in order toreconstruct segment 102 a. At time T1, only two of the requestedfragments 391 a and 391 c have resulted in fragment reception, and wereplaced 394 a, 394 c in the buffer 398. Fragment 391 b has not yet beenreceived at time T1, but can still be received later, and therefore attime T1 the assembling device does not yet try to complete the missingfragment with an additional fragment request. Instead, it proceeds andrequests fragments associated with segment 102 b. At time T2, all of thefragments requested for segment 102 b have arrived, and have been placed395 a, 395 b, 395 d in the buffer 398. Prior to time T2, the assemblingdevice transmits additional requests for fragments associated withsegment 102 c, and at time T3 two out of the requested fragments havearrived, and have been placed 396 b, 396 c in the buffer 398. At timeT3, the assembling device realizes that the chances on receiving thepreviously requested fragment 391 a (associated with segment 102 a) aretoo small. This may be concluded, for example, as a long time havingelapsed since the request, or by receiving a message from afractional-storage server saying it is too loaded to respond with afragment. Either way, the assembling device chooses to request anadditional fragment 391 d, instead of the previously requested 391 b. Attime T4, the additional request is met with the reception of fragment391 d, and with its placement 394 d in the buffer 398. At time T5, thethird fragment previously requested for segment 102 c has finallyarrived and has been placed 396 a in the buffer 398, so there is no needto complement with an additional fragment request. At time T5 allfragments needed to reconstruct segments 102 a to 102 c are stored inthe buffer 398. It is noted that only one additional fragment requestwas needed in order to account for the lack of reception of fragment 391b, and that this additional fragment request was issued after consequentfragments had already been requested for consequent segments.

In one embodiment, significant communication latency and/or otherlatencies between requesting and receiving a fragment exists. Asignificant latency may result in a case where the average latency inresponding to fragment requests is in the order of magnitude of thetotal transmission time of all fragments needed to reconstruct asegment. As an example, if a segment needs 64 fragments of 1500 Byteseach to be reconstructed, and the assembling device has a 1.5 Mpbsincoming connection, then it takes about (64(fragments)×1500(bytes perfragment)×8(bits per byte))/1.5 Mbps=0.512 seconds to transmit thefragment via the incoming connection. If the average latency is 0.2seconds (which is within the order of magnitude of 0.512 seconds), thenfrom the time of requesting the first fragment to the time all fragmentshave arrived, a period of no less than 0.512+0.2=0.712 seconds mayelapse. If the process takes 0.712 seconds, the resulting effectiveincoming throughput will be only (64(fragments)×1500(bytes perfragment)×8(bits per byte))/0.712(seconds)=1.07 Mbps, which issignificantly less than the potentially 1.5 Mbps. In a case where somefragments are lost, and need to be requested again, the total time forsegment retrieval may reach as high as 0.512+0.2+0.2=0.912, and theeffective incoming throughput down to only 842 Kbps. The significantlatency therefore adversely affects the effective incoming throughput.The effective throughput can be made to approach the incoming bandwidthavailable to the assembling device by utilizing the above-describedfragment retrieving process comprising the two sub-processes ofrequesting fragments and complementing the failures. In this case, thefirst sub-process can be made to result in an average targeted fragmentreception throughput, and span multiple segments, without handling thelost fragments. The second sub-process can then complement withadditional needed requests, approximately per each fragment request thatseems not to result in an actual fragment reception. According toanother view, the first sub-process is an open loop retrieval process,in which the assembling device does not wait to check whether enoughfragments have arrived per segment. And the second sub-process is theprocess, which closes the loop on fragments arrival, in order to makesure that every segment has enough fragments to enable reconstruction.

In one embodiment, an assembling device may control the erasure-codedfragment reception throughput by controlling the rate of fragmentrequest. For example, each of n fragments has a known size S1 to Sn.Therefore, issuing n requests over a period of T will result in anaverage fragment reception throughput of (S1+S2 . . . +Sn)/T. In oneexample, if each fragment is 1500 Bytes, and 64 fragment requests areissued over a period of 0.5 seconds, then the average expected fragmentarrival throughput is (64×1500×8)/0.5=1.53 Mbps. The fragment requestsdo not need to be uniformly spread over the period of 0.5 seconds,although such a spread may result in a more stable throughput, whichmeans that less communication buffering will be needed. Using theabove-described rate-control technique may result in one or more of thefollowing: retrieving the content at a target fragment receptionthroughput; preventing communication buffer spill at the last milenetwork resulting from uncontrolled fragment requests; and/or reducingfragment loss due to averaging the fragment traffic.

In one embodiment, an assembling device transmits aggregated messages toa relay server, including the number of fragments needed per certainsegment, but without identifying the storage servers from whichfragments are to be requested. The relay server selects the appropriatestorage servers to which the fragment requests are to be transmitted,and transmits discrete or aggregated fragment requests, corresponding tothe number of fragments requested by the assembling device, to theselected storage servers. The storage servers receive the fragmentrequests from the relay server, and transmit the requested fragment tothe assembling device. The relay server may select the storage serversaccording to one or more criteria, as long as the selected storageservers store relevant fragments. Optionally, the relay server forwardsthe address of the assembling device to the selected storage servers,and/or adds the address of the assembling device to the fragmentrequests transmitted to the selected servers, in order to enable thestorage servers to transmit the fragment response to the assemblingdevice.

In one embodiment, shifting the process of selecting the storage serversfrom the assembling device to the relay server enables the design of arelatively thin and simple assembling device, having a relatively simplesoftware, since all the assembling device has to decide in order toissue an aggregated fragment request to the relay server is how manyfragments it needs per segment and, optionally, when it needs them.

In one embodiment, an assembling device transmits aggregated messages toa relay server, comprising general information regarding a portion ofstreaming content for which fragments are needed. Optionally, theportion of the streaming content comprises several consecutive segments.In one embodiment, the portion is defined by a starting point and anending point within the streaming content, and the relay server usesthese points to determine the actual segments comprising the portion.Then the relay generates and transmits the corresponding fragmentrequests to the relevant storage servers.

FIG. 17 illustrates one example of a fractional-storage systemcomprising servers 699 a to 699(N) having a bandwidth capability 681. Inother words, no server can send data at a rate higher than 681.Assembling device 661 can select from which servers to obtainerasure-coded fragments for reconstruction of a segment. In one example,each server stores one relevant, unique, erasure-coded fragment.Therefore, from the N servers storing N possible unique fragments, theassembling device needs only K erasure-coded fragments for completereconstruction of the segment (K<N). Since it is not important which Kfragments from the N are retrieved, the assembling device may retrievefrom the least loaded servers, so as to keep the load between thedifferent servers balanced. When many assembling devices assemblecontents in parallel, and since all assembling devices can select theleast loaded servers, the end effect is that the load on the servers isbalanced, with the potential for most servers to approach their maximalbandwidth capabilities. Optionally, that load balancing is achievedwithout significant coordination between the servers.

In the example of FIG. 17, assuming that K=3, the assembling device 661may select servers 699 b, 699(N−1), and 699 a for fragment retrieval, asthey have the lowest load of all N servers. Servers 699 c and 699(N), asan example, will not be chosen, as they have relatively higher loads.

The assembling device may select the least loaded servers using anyappropriate method, such as, but not limited to (i) accessing a centralcontrol server having data about the load conditions on the variousservers, or (ii) periodically querying the various servers on their loadconditions.

In one embodiment, instead of, or in addition to, selecting the leastloaded servers, the assembling device 661 tries a random set of Kservers from the N, and retrieves erasure-coded fragments from allservers reporting a load below a threshold, while higher loaded serverswill be replaced by least loaded servers from the possible N servers.The end result is that the server array is balanced because the Kerasure-coded fragments are retrieved from servers loaded below thethreshold.

In one embodiment, the assembling device does not know which of theservers store erasure-coded fragments related to the content to beretrieved, but the assembling device knows over how many servers (fromthe total number) the erasure-coded fragments are distributed.Therefore, the assembling device compensates for the infertile requestsby enlarging the number of requests for erasure-coded fragments.Optionally, the requested servers are selected based on approximatelyrandom algorithm.

FIG. 18 illustrates one embodiment of different servers 698 a to 698(N)having different bandwidth capabilities of 683 a to 683(N)correspondingly. Assembling device 661 selects from which K servers, outof the possible N, to retrieve the fragments for segment reconstruction,wherein each server may have different unutilized bandwidth anddifferent bandwidth capability. When many assembling devices assemblecontents in parallel, while rejecting servers with a high load, the endeffect is that the server array is approximately balanced and mostservers can approach their maximal bandwidth capabilities. In oneembodiment, the server array is balanced by enabling many assemblingdevices to select the least loaded servers. In the example, and assumingthat K=3, servers 698 a, 698(N−1) and 698(N) will be selected, as theyhave the highest unutilized bandwidth. In another example, the servershaving the highest percentage of unutilized bandwidth will be selected.

In one embodiment, servers 698 a to 698(N) represent completelydifferent types of server hardware, operating systems and capabilities,all put together in an array, and achieving load balance without theneed for significant inter-server coordination. In one example, thefragments are distributed to at least two different classes of servers;the first class comprises high bandwidth CDN servers directly connectedto the Internet backbone, and the second class comprises lower bandwidthCDN servers not directly connected to the Internet backbone.

In one embodiment, the servers are selected for fragment retrievalaccording to their unutilized fragment delivery bandwidth. For example,the servers report their unutilized bandwidth, and the assemblingdevices, or a control server, obtain the report and decide which serversto use for fragment delivery based on the unutilized bandwidth of eachserver.

In one embodiment, the servers are selected for fragment retrievalaccording to their ability to support additional fragment delivery load.For example, the servers report their ability to support additionalfragment delivery loads. And the assembling devices, or a controlserver, obtain the report, and select the servers that report an abilityto support additional fragment delivery loads.

In one embodiment, the assembling device, or a control server, looks fora pool of servers that may be used as replacements for servers that areloaded to a degree that does not allow continuation of fragmentdelivery. For example, the assembling device looks for potentialunloaded servers, while retrieving fragments from other servers. Theassembling device may sample relevant servers approximately randomly,and/or according to indications from a control server. The samplingprocess may comprise querying the potential server for load information,or measuring the latency or latency variance to the servers in order toestimate the current load on the server.

In one embodiment, it is desired to replace one or more servers by otherservers for the delivery of erasure-coded fragments, wherein thereplacement servers are selected using a second criterion from a pool ofservers identified using a first criterion. For example, the firstcriterion for identifying the pool of replacement servers compriseslooking for servers capable of increasing their fragment deliverythroughputs, and the second criterion for selecting the replacementservers from the pool comprises selecting the best latency responseserver from the pool. In one example, the first criterion is a latencycriterion, and the second criterion is a load criterion. In anotherexample, the first criterion is a latency criterion, and the secondcriterion is a latency variance criterion. In another example, thesecond criterion is an approximately random selection. In oneembodiment, a server selected using the second criterion is compared tothe server to be replaced based on the second criterion. For example,the second criterion is latency, and the replacing server, selected fromthe pool, has a smaller latency than the server it replaces.

In one embodiment, the server to be replaced is identified by comparingthe actual performance level of the server with a threshold performancelevel. For example, when the compared performance is latency, a serverhaving response latency above a certain threshold is replaced. Inanother example, the compared performance is the load on the server,which may be measured in terms of the amount of the unutilized fragmentdelivery bandwidth, or in terms of the percent of the server'sunutilized fragment delivery bandwidth, or measured by any otherappropriate technique.

In some embodiments, the assembling devices use a fragment pull protocolto retrieve the fragments and approach the servicing servers. In someembodiments, the assembling devices use a push protocol to obtain thefragments and approach the servicing servers, possibly by obtainingmultiple sub-transmissions comprising fragment sequences.

FIG. 19 illustrates one embodiment of a fractional-storage system.Assembling device group 661 g obtain erasure-coded fragments from theservers, such that the resulting outgoing bandwidth utilizations of eachserver in the array is 682 a to 682(N) correspondingly. FIG. 20illustrates a case where server 698 b has failed, its bandwidthcapability 682 b 1 is zero, and is therefore unable to provideerasure-coded fragments. The assembling devices from group 661 g, whichpreviously obtained fragments from server 698 b, may attempt to accessit again for additional fragments, but are now unable to get a response.These assembling devices therefore obtain fragments from alternativeservers. The end effect is that bandwidth 682 b is now loaded on thestill available servers, such that the total bandwidth 682 a 1 to682(N)1 approximately increases by a total amount equal to 682 b,optionally with no inter-server coordination, and simply by the factthat each assembling device selects alternative available servers forobtaining fragment on-the-fly. In one example, instead of obtaining fromserver 682 b 1, the assembling devices obtain from the least loadedavailable servers. In one embodiment, a control server selects thealternative server/s for the assembling devices. In one embodiment, theassembling devices use a fragment pull protocol to obtain the fragments,and approach the alternative servers. In one embodiment, the assemblingdevices use a push protocol to obtain the fragments, and approachalternative servers, possibly by obtaining multiple sub-transmissionscomprising fragment sequences. In this case, the sub-transmissions ofthe faulty server are discontinued and compensated for by othersub-transmissions from the alternative servers.

FIG. 21 illustrates an example similar to FIG. 20 with the differencethat servers 698 a, 698 b, and 698 c to 698(N) reside within, or getserviced via, first, second, and third Internet backbone providers 300j, 300 i, and 300 h correspondingly. The group of assembling devices 661g is connected to the Internet via network 300 k, which has access toall three backbones, such that communication between the assemblingdevices and servers 698 a to 698(N) pass via at least one of thebackbones, or more. If server 698 b is made unavailable to theassembling devices, optionally not due to a server failure, but ratherdue to congestion or a failure of the second Internet backbone provider300 i, assembling devices 661 g compensate for the lost bandwidth byswitching to the available servers on-the-fly. In one embodiment,networks 300 h, 300 i, and 300 j, are different physical sub-nets of onenetwork connected to the Internet. In one embodiment, the assemblingdevices are connected to networks 300 h, 300 i, and 300 j, via network300 k, and then via one or more Internet Exchange Points (“IX/IXP”).

FIG. 22 illustrates a few examples of retrieving fragments according tolocality. In one example, the fractional-storage servers are connectedto a data network or networks comprising the routers 201 to 209.Assembling devices 235, 237, and 238 are connected to the same datanetwork or networks, and K=3, meaning that any assembling device needsto obtain 3 erasure-coded fragments per segment from optionally 3different servers out of the 10 in order to successfully reconstruct thesegment.

Each assembling device tries to obtain erasure-coded fragments fromfractional-storage servers that are closest to it topologically. In oneembodiment, the topological distance is a function of the number ofseparating routers. Assembling device 238 can select three servers fromgroups 242, 248 or 249. According to the minimal path criterion, itretrieves the erasure-coded fragments from servers 399 h to 399 i ofgroup 248, since they are only one router 208 away. Groups 242 and 249are three (208, 202, 203) and five (208, 202, 203, 201, 209) routersaway, and are therefore not selected for retrieval. Similarly, device237 selects three servers out of group 242, and device 235 can selectany three servers from groups 242 and 249, since both are located fourrouters away.

In one embodiment, if topologically close servers do not respond to theassembling device, or report a bandwidth limitation, the assemblingdevice will attempt to obtain an erasure-coded fragment from the nexttopologically closest server.

In one embodiment, an assembling device attempts to obtain erasure-codedfragments from servers featuring the lowest latency. Upon no response,for whatever reason, the assembling device will attempt to retrieve fromthe next lowest latency server. In one embodiment, the assembling deviceobtains information regarding the unutilized fragment deliverybandwidths of servers, and then attempts to retrieve from the lowestlatency servers out of the servers having enough unutilized bandwidth.In one embodiment, the assembling device obtains information regardingthe unutilized fragment delivery bandwidths of the servers, and thenattempts to retrieve from the topologically closest servers out of theservers having enough unutilized bandwidth.

Still referring to FIG. 22, in one embodiment the assembling devicesselect servers according to a latency criterion, such as selectingservers with the shortest time between fragment request and fragmentdelivery, or selecting servers having latency below a dynamic or staticthreshold. Assembling device 237 assembles content from servers 399 c,399 f, 399 g, and assembling device 235 assembles content from servers399 b, 399 c, 399 g (both use a mixture of servers from groups 242 and249). At a certain point in time, router 209 becomes congested orblocked, and prevents the erasure-coded fragments from servers 399 b and399 c from arriving at assembling devices 235 and 237, or causes thefragments to arrive with an increased delay. Therefore, assemblingdevice 235 switches to three servers of group 242, and assembling device237 switches from server 399 c to server 399 e.

In one embodiment, the assembling device selects fractional-storageservers according to the following criterion: first, servers withadequate unutilized fragment delivery bandwidth are considered, then outof these, those with latency below a threshold are considered, and outof these, the servers with minimal topological routing path areselected.

In some embodiments, the assembling devices use a fragment pull protocolto retrieve the fragments, and approach servers having low latency orlow hop count as compared to other servers. In some embodiments, theassembling devices use a push protocol to retrieve the fragments, andapproach servers having low latency or low hop count as compared toother servers, optionally by obtaining multiple sub-transmissionscomprising fragment sequences.

In one embodiment, a plurality of unsynchronized retrieving assemblingdevices, which optionally use fragment pull protocol, choose the leastloaded servers from which to retrieve the erasure-coded fragments.Optionally, the servers have almost no inter-communication between themand the load balancing calculation is performed by the retrievingassembling devices. Because the assembling devices can select the leastloaded servers, the assembling devices manage the load balancing. Whenthe erasure-coded fragments stored by the servers are uniqueerasure-coded fragments, the retrieving assembling device may retrieveerasure-coded fragments from any relevant server. Therefore, it may beenough for the retrieving assembling device to have indication of theload on its targeted servers, and retrieve enough erasure-codedfragments from the least loaded servers.

In one embodiment, a server signals the retrieving assembling devicethat it is close to its bandwidth limit and the assembling devicesearches for an alternative server. Optionally, the assembling deviceselects the server according to one or more of the following parameters:locality, cost, latency, or reliability. In one embodiment, the serversregister their loads on a central server, and the assembling deviceselects the server to retrieve from, from the registered servers. In oneembodiment, a central server, holding the loads of the various servers,determines for the assembling devices from which server to retrieve theerasure-coded fragments.

In one embodiment, assembling devices measure the latency of thedifferent servers in responding to fragment requests, and then use thelatency information to estimate the loads on the servers. In oneexample, a high latency may indicate a high load on the server.

In one embodiment, the topological router hop count between anassembling device and fragment delivering servers is used to estimatethe latency of the servers in responding to fragment requests.

In one embodiment, the latency of fragment delivering servers inresponding to fragment requests by an assembling device is used toestimate the topological router hop count between an assembling deviceand the servers.

In one embodiment, the assembling devices perform several latencymeasurements for the different servers in responding to fragmentrequests, and then use the latency variance information to estimate theloads on the servers. In one example, a high latency variance maysuggest a high load on server.

In one embodiment, fractional-storage servers, from which the fragmentsare obtained for reconstructing a segment, are selected based on anapproximately random selection algorithm from all of the servers storingthe relevant fragments. In one example, an approximately randomselection algorithm weighted according to the unutilized bandwidth ofthe servers is used for the approximately random selection of servers.The weighted random selection algorithm assigns servers with selectionprobabilities proportional to the amount of unutilized bandwidth forfragment delivery in each of the servers, such that the probability toselect a server having a larger amount of unutilized bandwidth is higherthan the probability to select a server having a lower amount ofunutilized bandwidth.

The following embodiments describe processes for on-the-fly selectionand re-selection of fractional-storage servers from which to obtainerasure-coded fragments.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments, based on the unutilized bandwidth of the servers,includes the following steps: (i) accessing data regarding serversstoring relevant fragments (referred to as the relevant servers); (ii)accessing data regarding the unutilized bandwidth of the relevantservers. Optionally, the data is received by the assembling device fromthe relevant servers; and (iii) obtaining fragments from enough of therelevant servers having approximately the highest unutilized bandwidth;or obtaining fragments from enough of the relevant servers selectedrandomly and having unutilized bandwidth above a certain threshold.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments, based on latency, includes the following steps: (i)accessing data regarding the relevant servers; (ii) accessing dataregarding the latencies from the relevant servers to the assemblingdevice; and (iii) obtaining fragments from enough of the relevantservers having the lowest latencies; or obtaining fragments from enoughof the relevant servers selected randomly and having latencies below acertain threshold.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments, based on bandwidth and latency, includes thefollowing steps: (i) accessing data regarding the relevant servers; (ii)accessing data regarding the unutilized bandwidth of the relevantservers; (iii) identifying more than enough relevant servers having themost unutilized bandwidth; or randomly identifying more than enoughrelevant servers having unutilized bandwidth above a certain threshold;(iv) accessing data regarding the latencies from the identified serversto the assembling device; and (v) obtaining fragments from enough of theidentified servers having the lowest latencies; or obtaining fragmentsfrom enough of the relevant servers selected randomly and havinglatencies below a certain threshold.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments, based on latency and bandwidth, includes thefollowing steps: (i) accessing data regarding the relevant servers; (ii)identifying more than enough relevant servers having latencies to theassembling device below a certain threshold; or randomly identifyingmore than enough relevant servers having latencies to the assemblingdevice below a certain threshold; (iii) accessing data regarding theunutilized bandwidth of the identified servers; and (iv) obtainingfragments from enough of the identified servers having the highestunutilized bandwidth; or obtaining fragments from enough of the relevantservers selected randomly and having the highest unutilized bandwidth.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments, based on locality, includes the following steps:(i) accessing data regarding the relevant servers; (ii) accessing dataregarding the network topology distance (locality) from the relevantservers to the assembling device; and (iii) obtaining fragments fromenough of the topologically closest relevant servers; or obtainingfragments from enough of the relevant servers that are located in thesame sub-network as the assembling device, or located in the closestsub-networks.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments, based on bandwidth and locality, includes thefollowing steps: (i) accessing data regarding the relevant servers; (ii)accessing data regarding the unutilized bandwidth of the relevantservers; (iii) identifying more than enough relevant servers having themost unutilized bandwidth; or randomly identifying more than enoughrelevant servers having unutilized bandwidth above a certain threshold;(iv) accessing data regarding the network topology distance from therelevant servers to the assembling device; and (v) obtaining fragmentsfrom enough of the topologically closest relevant servers; or obtainingfragments from enough of the relevant servers that are located in thesame sub-network as the assembling device, or located in the closestsub-networks.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments, based on latency and locality, includes thefollowing steps: (i) accessing data regarding the relevant servers; (ii)identifying more than enough relevant servers having latencies to theassembling device below a certain threshold; or randomly identifyingmore than enough relevant servers having latencies to the assemblingdevice below a certain threshold; (iii) accessing data regarding thenetwork topology distance from the relevant servers to the assemblingdevice; and (iv) obtaining fragments from enough of the topologicallyclosest relevant servers; or obtaining fragments from enough of therelevant servers that are located in the same sub-network as theassembling device, or located in the closest sub-networks.

In one embodiment, a method for selecting enough new servers from whichto obtain fragments is based on bandwidth, latency, locality, and,optionally, one or more additional relevant parameters. The method mayweigh the different parameters in various ways, all of them are intendedto be covered by the embodiments. For example, the method may includethe following steps: (i) accessing data regarding the relevant servers;(ii) receiving data regarding the unutilized bandwidth latencies to theassembling device, and topology distances to the assembling device;(iii) weighting the received data and identifying a quantity of the mostproper relevant servers, which can provide enough fragments toreconstruct content; and (iv) obtaining the fragments from theidentified servers. In another example, the method may include thefollowing steps: (i) accessing data regarding the relevant servers; (ii)identifying a set of more than enough relevant servers having the mostunutilized bandwidth; or randomly identifying a set of more than enoughrelevant servers having unutilized bandwidth above a certain threshold;(iii) from the set, identifying a sub-set of more than enough relevantservers having latencies to the assembling device below a certainthreshold; or randomly identifying more than enough relevant servershaving latencies to the assembling device below a certain threshold; and(iv) obtaining fragments from enough of the topologically closestrelevant servers out of the sub-set; or obtaining fragments from enoughof the relevant servers out of the sub-sets, which are located in thesame sub-network as the assembling device, or located in the closestsub-networks.

In one embodiment, a server may be loaded to a point that it isapproximately unable to transmit additional fragments as a response tonew fragment requests or new sub-transmission requests. The server mayalso be too loaded to continue transmitting fragments to its currentlyserved assembling devices. In one example, these cases result from oneor more of the following conditions: (i) server hardware limitation,such as CPU power or memory bus constraints, which prevents it fromdelivering fragments beyond a certain throughput, (ii) outgoingcommunication link limitation, such as a fixed-bandwidth line, whichprevents the server from transmitting fragments beyond a rate that canbe supported by the line, (iii) sharing of an outgoing communicationline with other servers, and the other servers utilizing the shared lineto a point that lowers the bandwidth available for fragmenttransmission, and (iv) sharing the fragment storage and transmissionsoftware together with other applications on one physical server, andthe other applications consuming CPU, memory, or communication resourcesto a point that affects the ability of the fragment storage andtransmission software to respond to fragment or sub-transmissionrequests.

In some embodiments, approximately random selection offractional-storage servers is utilized for dealing with changes innetwork conditions, such as packets loss and/or server failure, withoutaffecting the user experience, and optionally without prior knowledge ofthe type of the change in network condition. Optionally, newerasure-coded fragments are requested from the randomly selected serversinstead of failed requests. Optionally, failed servers are replaced withother servers. Optionally, the combination and/or the number offractional-storage servers from which the fragments are obtained changesover time. Optionally, the number of redundant fragment requests changesover time.

In one example, a constant packet loss condition causes a constantfragment loss condition, which means that a certain percentage offragments fail to be obtained by the assembling device. In this case, anapproximately random selection of new servers may solve the problem, notnecessarily because of the randomness of the selection (a generalfragment loss condition may affect all servers), but simply because itgenerates more fragment requests to compensate for the loss, resultingin an increased fragment-delivery throughput that approximately levelsat an average steady state value of:(Nominal_Throughput/(1−Fragment_Loss_Ratio)), wherein theNominal_Throughput is the fragment-delivery throughput resulting when nopackets are lost, and the Fragment_Loss_Ratio is the(fragment_lost/fragments_sent) ratio, which is a parameter thatincreases monotonically with the packet-loss. In another example, thefailure is specific to one or more servers, and the approximately randomselection of new servers finds new servers having lower failure ratios.In this case, the random selection solves the problem, since trying toretrieve again from problematic servers may have no positive effect. Theabove two examples demonstrate how a single selection strategysuccessfully copes with different types of failures, while resulting ina different behavior according to the type of failure (differentresulting fragment delivery rates for example), and all that withoutprior knowledge of the exact nature of the failure. In another example,the servers are deployed over multiple networks and the communicationfault comprises a failure of one of the networks causing related serversto be inaccessible. As a solution, the assembling device approximatelyrandomly reselects the servers until it communicates with enoughaccessible servers to reconstruct a segment. Other examples arepossible, in which an unknown failure is correctly handled byapproximately random on-the-fly server selection.

In one embodiment, different servers receive different weightsproportional to their bandwidth. For example, the higher the bandwidthcapability of the server, the higher the server coefficient; the higherthe server coefficient, the higher the probability of selecting theserver by an assembling device. In one embodiment, selecting the serversapproximately randomly enables the fractional-storage system to operatewell when the assembling devices do not know the load on at least someof the servers.

In one embodiment, the approximately random selection of serversproduces a set of source servers from which erasure-coded fragments areretrieved using a fragment pull protocol. In another embodiment, theapproximately random selection of servers produces a set of sourceservers from which erasure-coded fragments are retrieved using apush-protocol. In this case, multiple sub-transmissions may be used totransport the fragments from multiple servers to an assembling device.When new server sources are randomly selected instead of others, theassembling device may end the sub-transmissions associated with thereplaced servers, and initiate new sub-transmissions from the replacingservers, optionally from the point that the terminated sub-transmissionswere interrupted.

In one embodiment, the approximately random server selections are madefrom the servers not currently servicing the assembling device. In oneembodiment, the approximately random server selections are made from allservers storing relevant fragments, including the server(s) thatserviced the assembling device before being identified as problematic.

In one embodiment, approximately random reselections of servers areperformed occasionally, even if all currently servicing servers arefunctioning correctly. In this case, the assembling device may select afew servers from the current set, to be randomly replaced. In oneembodiment, functioning servers are kept throughout several segmentretrieval cycles, and potentially for the entire delivery cycle of asegmented content.

In one embodiment, a method for reselecting one or morefractional-storage CDN servers on-the-fly, comprising: pullingerasure-coded fragments from the servers; estimating the servers' load,latency, network congestion, and packet loss; and operating a fuzzyalgorithm on the estimations in order to replace at least one of theservers with at least one other fractional-storage server. Optionally,the method further comprising operating the fuzzy algorithm based onmeasurements of many assembling devices and recommendations receivedfrom a central server. Optionally, the method further comprisingreplacing the servers quickly after missing a fragment. And optionally,the fuzzy algorithm weighs many possible solutions and converges to asufficient one.

In one embodiment, different quantities of erasure-coded fragments aregenerated per different segments. In one embodiment, some segments storedata that is considered more important than data stored in othersegments, and relatively more erasure-coded fragments are generated fromthe segments storing the more important data than from the segmentsstoring the less important data.

In one example, a compressed video content is segmented into segmentsstoring i-frames and segments storing p-frames. Optionally, all segmentsare approximately of the same size, and more erasure-coded fragments aregenerated from the segments storing the i-frames than from the segmentsstoring the p-frames. Alternatively, the segments storing the i-framesare shorter than the segments storing the p-frames, and approximatelythe same quantity of erasure-coded fragments are generated from thesegments storing the i-frames and from the segments storing thep-frames.

In one example, a DCT content is segmented into segments storing lowfrequencies and segments storing high frequencies. Optionally, allsegments are approximately of the same size, and more erasure-codedfragments are generated from the segments storing the low frequenciesthan from the segments storing the high frequencies, where in addition,the size of the erasure-coded fragments generated from the segmentsstoring the low frequencies is smaller than the size of theerasure-coded fragments generated from the segments storing the highfrequencies. Alternatively, the segments storing the low frequencies areshorter than the segments storing the high frequencies, andapproximately the same quantity of erasure-coded fragments are generatedfrom the segments storing the low frequencies and from the segmentsstoring the high frequencies.

In some embodiments, the content is segmented into a plurality ofsegments to enable beginning to play the content as it is beingobtained, and optionally enable trick play. The different segments mayor may not be of the same size.

The following embodiments discuss different methods for segmenting thecontent. In one embodiment, at least one portion of the content issegmented into multiple segments in sizes within a first size range, andthe remainder of the content is segmented into a plurality of segmentsin sizes within a second size range (additional size/s may be addedsimilarly). The sizes included in the second size are larger than thesizes included in the first size range. Pluralities of erasure-codedfragments are generated from each of the segments. The segments of sizeswithin the first size range are better suited for fast retrieval, andthe segments of sizes within the second size range are better suited forhigh-gain storage. In one example, the segments in sizes within thefirst size range belong to approximately the beginning of the content.In one example, the segments in sizes within the first size range belongto locations within the content requiring trick play access. In oneembodiment, the segments of the first type are encoded into fewerfragments than the segments of the second type. This allows a fastretrieval of the shorter segments.

In one example, the content 100 is a 1 GByte encoded H.264 file, storinga 2-hour motion picture, and is segmented into approximately 10,000segments of approximately 100 Kbytes each. In another example, thecontent 100 is a 4 MByte web-site information (HTML, FLASH, or any othercombination of information that encodes the presentation of a website),and is segmented into 4 segments of approximately 1 MByte each.

In one example, the content supports streaming presentation, and thesegments are small enough to enable presentation shortly after beginningthe reception of the first segment(s). For example, each segment mayinclude 96 KByte, allowing a 5 Mbps receiver to download the segment inapproximately 0.2 seconds, and optionally begin the presentation shortlythereafter. In one embodiment, the time to play is reduced by segmentingcertain portions of the content into smaller segments, while theremaining portions are segmented into larger segments. A smaller segmentcan be retrieved faster, while a larger segment may be better optimizedfor storage gain and/or efficient transmission.

In one embodiment, the short segments are 96 Kbytes in size, and thelong segments are 960 Kbytes in size. The redundancy factors used forencoding short and long segments into fragments are 100 and 5respectively. 1500 Bytes fragments are used for both sizes. The shortsegments are therefore encoded into (96K/1500)×100=6,400 fragments, fromwhich only about 64 are needed for reconstruction, and the long segmentsare encoded into (960K/1500)×5=3,200 fragments, from which only about640 are needed for reconstruction. Short segments are reconstructed morequickly than long ones, as they require fewer fragments to be decoded.Optionally, each fragment is stored on a different server, resulting ina storage gain of 64 for short segments, and 640 for long segments.

FIG. 23 illustrates one example in which the content 100 is segmentedinto segments, such that the first segment 104 a is smaller than theconsecutive segment 104 b, which is smaller than following segments 104c and 104 d. In another example, the content 100 is segmented intosegments, such that the first several segments (e.g. 104 aa and 104 bb,which are the same size), are smaller than consecutive segments (e.g.104 cc and 104 dd, which are the same size).

FIG. 24 illustrates one example in which the content 100 is segmentedinto cyclic sets of successive segments increasing in size. For example,105 b is equal or larger in size than 105 a, and so on, up to segment105 d; 105 f is equal or larger in size than 105 e, and so on, up tosegment 105 h. In one example, segment 105 e is equal in size to segment105 a. Point 105EP represents the ending of the first set, and thebeginning of the second set.

In one embodiment, some of the content is segmented approximatelysequentially, and the rest non-sequentially. For example, dividing thebeginning of a video content sequentially in time, and the rest of thevideo content non-sequentially in time.

In one embodiment, a scalable, secured, moderate storage space, anddurable wide-area storage system is achieved by erasure-encoding thecontent, then chopping the encoded content into a plurality oferasure-coded fragments, and then distributing the erasure-codedfragments to the fractional-storage servers. In this embodiment, theretrieving clients may retrieve the stored erasure-coded fragments usingtheir maximum incoming bandwidth until approaching the server'saggregated bandwidth, the content may be any type of data file, and thechopping may be performed according to any reproducible method.

In one embodiment, segments are created on-the-fly, such as during alive event or when the content is made available to the segmentationprocess as an on-going stream. In one embodiment, the content supportsstreaming presentation, and the segments are of the small size, toenable content presentation shortly after beginning the reception of thefirst segment (or any other segment). In addition, the erasure-codedfragments are kept as small as possible, while still enabling efficienttransport over an IP network. For example, each erasure-coded fragmentis about 1500 Bytes and can be transported using one IP packet.

It is to be noted that streaming content may also be manifested as anintermediate product of a process. For example, in a case where a videocamera outputs erasure-coded fragments that can be decoded intostreaming content, the intermediate data from which the erasure-codedfragments are generated is considered to be streaming content (even ifthe video camera does not output that intermediate data). Moreover,streaming content may include: content that is produced and thenimmediately transmitted to a receiving server, content that is producedbut stored for any length of time before being transmitted to areceiving server, content that is transmitted to a receiving server andthen immediately sent from the receiving server to a client, contentthat is transmitted to a receiving server, then buffered for some timeat the receiving server and then sent from the receiving server to aclient, content that is solely played at a client, and content that ismanipulated or changed or reacted to at the client while a continuationof the content is still being played at the client.

FIG. 25 (without the fragments marked with dashed lines) illustrates oneexample of distributing the erasure-coded fragments to ‘M’ CDN servers399 a to 399(M), connected to a network 300. Encoded fragments 310 a to310(M) of a first segment are sent for storage in servers 399 a to399(M) respectively. Similarly, erasure-coded fragments 320 a to 320(M)of a second segment are sent for storage in servers 399 a to 399(M)respectively. In addition, other erasure-coded fragments associated withother segments of other contents, illustrated as erasure-coded fragments390 a to 390(M), are sent for storage in servers 399 a to 399(M)respectively. The number of unique erasure-coded fragments from eachsegment that are stored on the servers (399 a to 399(M)) is equal to Min this example, where M may be smaller than the maximum number ofunique erasure-coded fragments, meaning that only a subset of thepotential erasure-coded fragments are actually stored. It is alsopossible to store the maximum number of unique erasure-coded fragments,or store more than one unique erasure-coded fragment per segment perserver. The network 300 may be the Internet for example, or any otherdata network connecting multiple nodes, such as a private IP network, ora Wide Area Network (“WAN”). In one embodiment, the fragments markedwith dashed lines illustrate one example where (N−M) additional serversare added to the array, and (N−M) new unique erasure-coded fragments persegment per content (310(M+1) to 310(N), 320(M+1) to 320(N), and390(M+1) to 390(N)) are generated and added to the array. In oneembodiment, only M out of the maximum possible erasure-coded fragments(L) are actually generated for storage in the first place. In oneembodiment, when the additional N-M erasure-coded fragments are neededfor storage (e.g., when additional servers are made available), theremainder of the N-M erasure-coded fragments are actually generated. Anytime that additional unique erasure-coded fragments are needed, thisprocess of calculating the additional erasure-coded fragments isrepeated, up to the point that all L possible erasure-coded fragmentsare used.

In one embodiment, and especially when using rateless coding, L may bechosen as a sufficiently large number to account for any realisticfuture growth of the server array. For example, a segment of 96 Kbytesis expanded using a rateless code with a ratio of 1 to 2̂16 originalsymbols to encoded data, into an encoding symbol of potential size 6.29GBytes. Assuming a 1500 Bytes erasure-coded fragment size, thenpotentially 4.19 million unique erasure-coded fragments can begenerated. Now, it is safe to assume that for all practical uses, theserver array will not grow to more than 4.19 million nodes, and maycontain several thousands of servers, meaning that the encoded data canbe used in all cases where additional unique erasure-coded fragments areneeded, by generating new erasure-coded fragments out of the segment.Optionally, a server may store erasure-coded fragments for only some ofthe segments.

In one example of redundancy factor and storage gain (without thefragments marked with dashed lines), server 399 a stores onlyerasure-coded fragment 310 a from a first segment, erasure-codedfragment 320 a from a second segment, and erasure-coded fragment 390 afrom a third segment. Assuming that: (i) the segment size is 1024Kbytes; (ii) the segment is encoded using erasure code into a 4096 KByteencoded segment; (iii) the encoded segment is segmented into 256erasure-coded fragments of size 4096/256=16 KByte; and (iv) theerasure-coded fragments are stored on 256 servers (M=256); it turns outthat each server stores only a 1/64 portion of the original size of thesegment. This means that each server can manage with only 1/64 of thestorage requirements in comparison to a situation where it had to storethe entire segment. In addition, there are 256 erasure-coded fragmentsaltogether from each encoded segment, meaning that an assembling devicethat is assembling the erasure-coded fragments from the servers needonly select slightly more than 64 erasure-coded fragments in order tocompletely reconstruct the segment, and it can select whichever slightlymore than 64 erasure-coded fragments it desires out of the 256 possiblyavailable. The redundancy factor in this example is approximately256/64=4. All contents in this example enjoy a factor of 64 in storagegains, meaning that server 399 a, for example, stores only 1/64 of theinformation associated with the first segments and any additionalsegments belonging to other contents. In one example, each serversupports high volume storage of between about 500 GByte and 500 TBytes,optionally utilizing hard drive, Solid State Drive, or any other highvolume storage device(s). In these cases, each server may store manymillions of erasure-coded fragments, associated with millions ofsegments, belonging to hundreds of thousands of different contents, andpossibly more.

In one embodiment, new content initially encoded with a low redundancyfactor is distributed to an initial number of fractional-storageservers. As the content is distributed to more servers, additionalunique fragments are encoded and therefore the redundancy factorincreases. Optionally, as the content's popularity increases, and/or asthe load on the fractional-storage servers increases, the redundancyfactor is increased, and vice versa.

In one embodiment, multiple unique erasure-coded fragments per segmentof a new content are distributed to an initial number offractional-storage servers with a low storage gain (i.e. each serverstores multiple unique erasure-coded fragments per encoded segment). Asthe content is distributed to more fractional-storage servers, some ofthe erasure-coded fragments stored on the initial number offractional-storage servers are removed and thereby the storage gain isincreased. Optionally, as the demand for the content increases, thestorage gain is decreased, and vice versa.

FIG. 26 illustrates three examples (each depicted by one of the columnsA-C) of changing the redundancy factor according to the demand. Column Aillustrates one simplified example of a storage array including 16servers (1001 to 1016). Each server stores up to 2 differenterasure-coded fragments, and can service an erasure-coded fragmenttransmission bandwidth of up to B. Assuming three contents (#1, #2, and#3) processed to segments and erasure-coded fragments with a storagegain of 4.

Assuming content #1 is the most popular, and requires a peak bandwidthof 11×B. Since each server can service up to bandwidth B, at least 11servers are needed to service content #1 bandwidth requirements. Content#1 is therefore encoded into 11 unique erasure-coded fragments persegment, illustrated as group g1 of erasure-coded fragments stored onservers 1001 to 1011. Out of these 11 erasure-coded fragments, it issufficient to obtain slightly more than 4 erasure-coded fragments inorder to reconstruct a segment of content #1. Therefore, the resultingredundancy factor of the stored fragments associated with content #1 isapproximately 11/4=2.75. Content #2 requires less bandwidth, and manageswith a peak of 7×B. It is therefore encoded into 7 unique erasure-codedfragments per segment, illustrated as group g2 of erasure-codedfragments on servers 1010 to 1016. Therefore, the redundancy factor ofthe stored fragments associated with content #2 is 7/4=1.75. Content #3requires a peak bandwidth of 5×B, but for some reason (for example,being a more critical content), it is encoded into 14 erasure-codedfragments per segment, illustrated as group g3 of erasure-codedfragments on servers 1001 to 1009 and 1012 to 1016. Therefore, theredundancy factor of the stored fragments associated with content #3 is14/4=3.5. This concludes the storage availability of the servers in thisexample, as every server stores two erasure-coded fragments.

Column B illustrates an example where content #2 becomes more popularthan content #1, and therefore requires more bandwidth and hence more ofa redundancy factor. This is achieved by eliminating 5 erasure-codedfragments associated with content #1 that were previously stored onservers 1001 to 1005, and replacing them with 5 new unique erasure-codedfragments g4 associated with content #2. This brings the total number oferasure-coded fragments per segments of content #1 and #2 to 6 and 12respectively. In column C, new content #4 is stored on servers 1001 to1003 and 1014 to 1016 (illustrated as g5), by eliminating 3erasure-coded fragments of content #1 and 3 erasure-coded fragments ofcontent #2.

Throughout the examples of FIG. 26, a record of “what erasure-codedfragments are stored where” may be: (i) kept in each of the servers 1001to 1016. In this case, when an assembling device is assembling content#2, it will send a query to servers 1001 to 1016, asking which one isstoring erasure-coded fragments of content #2; (ii) kept in a controlserver. In this case, an assembling device will ask the control serverto send back a list of all servers storing erasure-coded fragments ofits required content.

By using a pull protocol or a push protocol with multiplesub-transmissions, the assembling device can obtain erasure-codedfragments from one, two or more different arrays of CDN servers and/orbandwidth amplification devices seamlessly.

FIG. 27 illustrates one embodiment in which fractional-storage servers399 a and 399 b are part of a server array. Fractional-storage servers399 a and 399 b store erasure-coded fragments 310 a and 310 b of a firstcontent, and erasure-coded fragments 320 a and 320 b of a secondcontent. Server 393 is a control server that manages a pool of twelveregistered bandwidth amplification devices surrounded by ellipse 599.One or more of the twelve bandwidth amplification devices may beassigned to one or more of the fractional-storage servers participatingin the array. In the initial stage, no assignments have been made, andthe twelve bandwidth amplification devices in pool 599 are ready toreceive instructions. Next, the control server 393 allocates sixbandwidth amplification devices of group 610 aa to server 399 a, and sixbandwidth amplification devices of group 610 bb to server 399 b.Registering the bandwidth amplification devices with the servers may beprocessed using any appropriate method. From groups 610 aa and 610 bb,three bandwidth amplification devices 610 a and 610 b are allocated tostore erasure-coded fragments 310 a and 310 b respectively (and,optionally, other erasure-coded fragments associated with consequentsegments of the content); and three bandwidth amplification devices 620a and 620 b are allocated to store erasure-coded fragments 320 a and 320b respectively (and, optionally, other erasure-coded fragmentsassociated with consequent segments of the content). After theseallocations have been made, fractional-storage server 399 a forwardserasure-coded fragment 310 a to group 610 a, and erasure-coded fragment320 a to group 620 a. Fractional-storage server 399 b forwardserasure-coded fragment 310 b to group 610 b, and erasure-coded fragment320 b to group 620 b. At the end of the allocation and forwardingprocess, the bandwidth amplification devices are ready to act asbandwidth amplifiers to the fractional-storage server array 399 a and399 b. Optionally, the allocation of bandwidth amplification devices tospecific contents is performed by either the control server 393, or eachfractional-storage server 399 a and 399 b.

It is noted that each bandwidth amplification device is not restrictedto storing and serving erasure-coded fragments associated with a singlecontent, and it is possible for each bandwidth amplification device tostore and serve multiple erasure-coded fragments associated withmultiple contents. The tradeoff in this case is that the moreerasure-coded fragments from more contents are stored and served, thelower the bandwidth amplification factor, since the rate of forwardingfragments from the server to the bandwidth amplification devicesincreases, while the outgoing bandwidth available for each bandwidthamplification device remains the same.

FIG. 28 illustrates one embodiment in which a fractional-storage server399 a stores erasure-coded fragments associated with four different VoDcontents 310 a to 340 a. Fractional-storage server 399 a controls twobandwidth amplification devices 428 and 429 (such as peer-to-peerclients, clients devices, STBs, game consoles, etc.). Server 399 adecides to forward the stored erasure-coded fragments as follows:erasure-coded fragments associated with the first two contents 310 a and320 a are forwarded to bandwidth amplification device 428, andsubsequent two contents 330 a and 340 a are forwarded to bandwidthamplification device 429. In this example, all of the erasure-codedfragments stored on the fractional-storage server 399 a are forwarded toits bandwidth amplification devices 428 and 429.

Up to a first bandwidth, some or all of the erasure-coded fragments maybe obtained from the bandwidth amplification devices 428 and 429 insteadof the fractional-storage server 399 a. Alternatively, up to a secondbandwidth, some or all of the erasure-coded fragments may be obtainedfrom the fractional-storage server 399 a and not from bandwidthamplification devices 428 and 429. In one example, a fractional-storageserver 399 a having a storage space of 200 Gbyte and storingerasure-coded fragments associated with 20,000 movies, can use 100bandwidth amplification devices, each having 2 GByte of FLASH memory,such that each bandwidth amplification device stores the erasure-codedfragments associated with about 200 movies.

FIG. 29 and FIG. 30 illustrate one embodiment, in which dynamicbandwidth amplification is achieved by allocating bandwidthamplification devices (e.g., client-devices, peers, etc.) according tocontent demand. The fractional-storage server 399 a stores two sets oferasure-coded fragments, 310 a and 320 a, associated with a firstcontent and a second content correspondingly. The erasure-codedfragments are either pre-stored on the fractional-storage server (e.g.,VoD), or are being received on-the-fly for storage and forwarding (e.g.,live event or broadcast effect). The fractional-storage server 399 a hasa group of 12 bandwidth amplification devices 599 allocated to it. Thefractional-storage server 399 a has allocated a maximum bandwidth B1 forserving the first content, a maximum bandwidth B2 for serving the secondcontent, and has a total bandwidth Bs for serving all contents, wherein(B1+B2)<Bs. As long as the bandwidth consumption of the first and thesecond contents are below B1 and B2 respectively, the fractional-storageserver 399 a can provide the demand and need not utilize any of thebandwidth amplification devices 599 for bandwidth amplification. Whenthe bandwidth used by the fractional-storage server 399 a to serve thefirst content either approaches or has already reached B1, it copies tofour of its controlled bandwidth amplification devices 598 all of theerasure-coded fragments associated with the first content or only theerasure-coded fragments that are still to be received on-the-fly. Group598 of the bandwidth amplification devices now acts as a bandwidthamplifier, and, as such, increases the bandwidth available to the firstcontent consumption above B1. The fractional-storage server 399 aselects the number of actual bandwidth amplification devices toparticipate in group 598 so as to bring back the first content bandwidthconsumed from server 399 to B1 or to a safe level below it.

FIG. 30 illustrates one example where the consumption of the firstcontent has increased again, and so the fractional-storage server 399 aincreases the number of bandwidth amplification devices participating inthe storage and the forwarding of the erasure-coded fragments associatedwith the first content to seven devices 598 a. In parallel, thebandwidth used by the fractional-storage server 399 a to serve thesecond content either approaches or has already reached B2. Therefore,the fractional-storage server 399 a copies to five of its controlledbandwidth amplification devices 597 a some or all of the erasure-codedfragments associated with the second content. Group 597 a now acts as abandwidth amplifier, and, as such, increases the bandwidth available tosecond content consumption above B2. Any further increase in demand forthe first or the second contents will require the fractional-storageserver 399 a to either obtain or request additional bandwidthamplification devices, to increase the bandwidth limits B1 and B2(optionally by increasing Bs), or to limit additional viewing requestsfor the contents. Any drop in the demand of either the first or thesecond contents will allow the fractional-storage server 399 a torelease bandwidth amplification devices, optionally in anticipation ofother contents that may need bandwidth boosting at some future point intime.

In one embodiment, when a CDN server receives a request for anerasure-coded fragment, it may supply the erasure-coded fragment orsupply an address of a bandwidth amplification device having an image ofthe requested erasure-coded fragment. Optionally, a bandwidthamplification device storing one erasure-coded fragment of a specificcontent also stores an image of some or all other erasure-codedfragments associated with the specific content (which are stored on thespecific CDN server). Alternatively, the bandwidth amplification devicestores unique erasure-coded fragments generated from the same segmentsused for generating the erasure-coded fragments stored on the specificCDN server. In these cases, the assembling device may approach thebandwidth amplification devices instead of the CDN server for therelevant erasure-coded fragments of the specific content until (i) theend of the content; (ii) a predefined time period elapses; (iii)receiving an appropriate message; or (iv) a combination of theaforementioned.

In one embodiment, an assembling device tries to obtain an erasure-codedfragment or sub-transmission from the relevant server, and if the serverdoes not have the necessary bandwidth to respond with fragment/s, theserver relays the fragment request/s to relevant bandwidth amplificationdevices. The relevant bandwidth amplification devices can then send thefragment/s directly to the assembling device.

FIG. 31 illustrates one embodiment in which server 399 a stores twoerasure-coded fragments 310 a and 320 a of two different contents.Server 399 a controls bandwidth amplification devices 420 a, 421 a, 520a, 521 a, and 522 a. The bandwidth amplification devices may be STBs,PCs, gaming consoles, or any other computing and storing devices thatcan connect to the network 300, and communicate with the server 399 a.Server 399 a sends erasure-coded fragment 310 a for storage in bandwidthamplification devices 420 a and 421 a. An assembling device 661 mayobtain an erasure-coded fragment 310 a from the server 399 a or from thebandwidth amplification devices 420 a or 421 a. Therefore, the maximalbandwidth available for reconstructing the erasure-coded fragment 310 ais now the sum of server 399 a bandwidth and the outgoing bandwidth ofthe two bandwidth amplification devices 420 a and 421 a.

Then, server 399 a sends erasure-coded fragment 320 a for storage inthree additional bandwidth amplification devices 520 a, 521 a and 522 a.Assuming all bandwidth amplification devices have a similar outgoingbandwidth, erasure-coded fragment 320 a is stored on more bandwidthamplification devices than erasure-coded fragment 310 a, and thereforethe content to be decoded from the erasure-coded fragment 320 a has moreavailable bandwidth for reconstruction than the content to be decodedfrom erasure-coded fragment 310 a.

In one embodiment, assembling device 661 tries to obtain anerasure-coded fragment from the relevant server (for example, fragment310 a from server 399 a), and if the server reports or sends anindication that it cannot provide the request, then the assemblingdevice 661 looks for the erasure-coded fragment in the bandwidthamplification devices, such as 420 a or 421 a. In another embodiment,server 399 a provides the assembling device 661 with possible addressesof bandwidth amplification devices, so that the assembling device candirectly approach one of them. The addresses may be provided when theassembling device 661 requests erasure-coded fragments from server 399a, or in advance, or by another network entity such as a controller ofthe server array.

The number of bandwidth amplification devices supporting eachfractional-storage server may be determined according to the demand forthe content to be reconstructed from the erasure-coded fragments. In oneexample, server 399 a receives erasure-coded fragment 310 a associatedwith a first segment and subsequent erasure-coded fragments associatedwith subsequent segments of the first content at an average rate of Bf(this can be a live broadcast event, for example, that is fragmentedon-the-fly for storage in a server array, to which server 399 abelongs). Server 399 a sends erasure-coded fragment 310 a and subsequenterasure-coded fragments (optionally, as soon as they are made available)to each one of bandwidth amplification devices 420 a and 421 a at theaverage rate of Bf. Assuming each bandwidth amplification device has anaverage available outgoing bandwidth of Bo, then the bandwidthamplification factor is (2×Bo)/(2×Bf)=Bo/Bf, meaning that each timeserver 399 a “sacrifices” Bf of bandwidth to forward erasure-codedfragments to bandwidth amplification devices; the content which is beingforwarded gains a potential benefit of Bo bandwidth back from thebandwidth amplification devices.

In one numerical example, streaming content is stored on an array offractional-storage servers having a storage gain of 256. The content hasa compressed stream rate of 1 Mbps, so that on average,fractional-storage server 399 a receives erasure-coded fragmentsassociated with the content at a rate of 1 Mbps/256=4 Kbps (=Bf). Now,server 399 a forwards the fragments to bandwidth amplification devices,each having an outgoing bandwidth of 384 Kbps (=Bo). The bandwidthamplification factor here is 384/4=96, meaning that if, for example,server 399 a (and similarly all other servers participating in thearray) has a bandwidth of 1 Gbps, and forwards the fragments to 250,000bandwidth amplification devices (meaning that the entire serverbandwidth is used for fragment forwarding), then the resultingaggregated available bandwidth for reconstruction of the content fromthe server 399 a and from the 250,000 bandwidth amplification devicescontrolled by server 399 a is 1 Gbps×96=96 Gbps. Other serversparticipating in the array may have similar available bandwidthresulting from additional bandwidth amplification devices. In oneexample, if 1024 servers participate in the array, and each servercontrols 250,000 bandwidth amplification devices (a total of about 250million bandwidth amplification devices), then the total availablebandwidth of the array is 96 Gbps×1024=about 100,000 Gbps=100 Tbps.

In one embodiment, the bandwidth amplification factor is the totalnumber of times a fragment forwarded to a bandwidth amplification deviceis delivered by the amplification device to assembling devices. In oneexample, one fragment belonging to VOD content is forwarded once to abandwidth amplification device by a server. This single fragment is thendelivered by the bandwidth amplification devices to 5,000 assemblingdevices during a period of one month. Therefore, the bandwidthamplification factor is 5,000.

In one embodiment, a control server decides which fragments belonging tospecific contents are to be replicated on bandwidth amplificationdevices. The decision may be based on information available to thecontrol server regarding additional bandwidth needed to supply excessdemand for some of the contents. In one embodiment, the control serveralso sends the fragments for replication on bandwidth amplificationdevices. Optionally, this can be the same control server used todistribute fragments among the fractional-storage servers.

In one embodiment, unique erasure-coded fragments can be distributedbetween two types of devices: (i) high bandwidth fractional-storageservers, such as CDN servers, and (ii) relatively low bandwidth andstorage devices acting as bandwidth amplification devices, such aspeer-to-peer (P2P) devices. Since the fragments distributed between thetwo types of devices are unique, any combination of devices, from bothtypes, can be used to obtain a decodable set of fragments, if thecombination of devices stores a decodable set of fragments. In oneembodiment, there are at least ten times more bandwidth amplificationdevices than high bandwidth servers, and the redundancy factor used indecoding the fragments is greater than 10. In this case, the servers canbe used all or most of the time, and the bandwidth amplification devicescan be used from time to time, according to bandwidth requirements, andaccording to the availability of the bandwidth amplification devices. Inone embodiment, the processes of obtaining a fragment from a server andfrom a bandwidth amplification device are essentially the same, and thefragments are essentially identical in construction and format. In oneembodiment, the high redundancy factor needed to support a large hybridarray of servers and bandwidth amplification devices is achieved usingrateless coding techniques.

FIG. 32 illustrates one embodiment of bandwidth amplification usingthree unique erasure-coded fragments (310 a, 310 a′, and 310 a″)generated from the same segment belonging to a first content, and 4unique erasure-coded fragments (320 a, 320 a′, 320 a″, and 310 a′″)generated from the same segment belonging to a second content.Fractional-storage server 399 a stores the two erasure-coded fragments310 a and 320 a, and sends the erasure-coded fragments 310 a′ and 310 a″for storage in bandwidth amplification devices 420 a and 421 a, and theerasure-coded fragments 320 a′, 320 a″, and 320 a′″ for storage inbandwidth amplification devices 520 a, 521 a, and 522 a. The maximalbandwidth available for obtaining erasure-coded fragment 310 a is nowthe sum of server 399 a bandwidth and the outgoing bandwidth of the twobandwidth amplification devices 420 a and 421 a. Decodable sets offragments may be obtained from any combination of the server and thebandwidth amplification devices. In one example, if two fragments areneeded for decoding, than any two fragments out of 310 a, 310 a′, and310 a″ may be obtained and decoded.

FIG. 33 illustrates one embodiment of hybrid Servers-P2P system using Nunique erasure-coded fragments 550 to 559, generated from the samesegment belonging to content. The fragments are partitioned into twogroups: server group comprising fragments 550 to 552; and P2P group,comprising fragments 553 to 559. Fragments belonging to the first group(550 to 552) are distributed among fractional-storage servers 560 to 562respectively. Fragments belonging to the second group (553 to 559) aredistributed among P2P devices (acting as bandwidth amplificationdevices) 563 to 569 respectively. In one example, N=30,003, and thereare close to 30,000 P2P devices. The following reconstruction modesassume that any three fragments are sufficient to reconstruct a segment:

In one reconstruction mode, the servers 560 to 562 have an aggregatedfragment-delivery bandwidth sufficient to supply all fragment demands ofassembling devices. In this case, the assembling devices obtainfragments only from the servers, and do not obtain any fragments fromP2P devices.

In another reconstruction mode, the servers 560 to 562 have anaggregated fragment-delivery bandwidth that is insufficient to supplyall fragment demands of assembling devices. In this case, the assemblingdevices obtain fragments from both the servers and the P2P devices. Anycombination of three fragments forms a decodable set of fragments.Selecting combinations for the different assembling devices, accordingto bandwidth availability of both servers and P2P devices, results in afragment delivery bandwidth that may approach the aggregated bandwidthof both the servers and P2P devices. According to the N=30,003 example,each P2P device has a fragment delivery bandwidth of 100 Kbps, and eachserver has a fragment delivery bandwidth of 1 Gbps. The three servers560 to 562 contribute 1 Gbps×3=3 Gbps of fragment throughput. The 30,000P2P devices contribute 100 Kbps×˜30,000=˜3 Gbps. The total fragmentdelivery bandwidth of the hybrid system therefore approximately equals 3Gbps+3 Gbps=6 Gbps. A Hybrid system comprising such a large number ofP2P devices needs a large redundancy factor. In the above example, theredundancy factor needed is approximately 30,003/3=10,000. Such largefactors can be realized using rateless codes. In the above example, thebandwidth amplification factor=the total possible bandwidth includingboth servers and P2P devices divided by the maximal possible bandwidthusing only the servers=6 Gbps/3 Gbps=2.

In one example, 1,000 fractional-storage CDN servers, each having afragment delivery bandwidth of 10 Gbps, are combined with 10 million P2Pdevices, each having a fragment delivery bandwidth of 1 Mbps, to producea (1,000×10 G)+(10M×1M)=20 Tbps streaming system.

In one embodiment, the bandwidth amplification devices are used tosupplement the streaming capabilities of the servers during peak trafficperiods. In this case, the aggregated bandwidth of the fixed bandwidthlines connecting the servers to the Internet need not account for thefull bandwidth demand. In one example, 1,000 CDN servers are connectedto the Internet via fixed bandwidth lines having a total capacity of 10Tbps. Demands above 10Tbps, which occur during a 3-hour period in theevening, are met by utilizing additional fragment delivery bandwidth ofP2P devices acting as bandwidth amplification devices.

In one embodiment, the content is streaming content comprisingapproximately sequential segments, and the assembling devices attempt toobtain decodable sets of fragments from the servers. Upon failure or anestimated failure to obtain the sets, the assembling devices obtain theadditionally needed fragments from one or more of the bandwidthamplification devices.

In one embodiment, the assembling devices receive an indication whetherto use the bandwidth amplification devices as additional fragmentsources to the servers. This indication may be received during periodsof high fragment demands, exceeding the servers' capacity. Theindication may be sent by the fractional servers or by a control server.

In some embodiments, the assembling devices may obtain the fragmentsfrom the servers and/or from the P2P devices using a fragment pullprotocol, a fragment pull protocol for high latency, and/or multiplesub-transmissions.

In one embodiment, the bandwidth amplification devices are located atthe user premises, and are connected to the Internet via ISPs. In oneembodiment, the fractional-storage servers are CDN servers directlyconnected to the Internet backbone, or located in close topologicalproximity to the backbone.

In one embodiment, the bandwidth amplification devices store both uniqueand replicated fragments.

In one embodiment, a distributed streaming system comprising:fractional-storage servers and bandwidth amplification devicesconfigured to store, correspondingly, a first and a second portions ofrateless-coded fragments associated with approximately sequentialsegments of streaming content, shortly after the segments areprogressively made available by a streaming source. Decodable sets offragments associated with the segments can be obtained fromapproximately any combination of servers and bandwidth amplificationdevices storing fragments associated with the segments shortly after thesegments are made available by the streaming source.

FIG. 34 illustrates one example, in which real-time content source 700is segmented 710 a, 710 b on the fly, each segment is rateless-coded onthe fly 720 a to 720(N−1) and 730 a to 730(N−1), a first portion of thefragments is distributed on the fly to fractional-storage servers 740a′, 740 b′ and a second portion of the fragments is distributed on thefly to P2P devices 750 a′, 750 b′ acting as bandwidth amplificationdevices. Each distributed fragment is unique. Any combination offragments out of fragments 720 a to 720(N−1) as an example, thatcomprise a decodable set of fragments, may be used to reconstructsegment 710 a on the fly. This can be done regardless of whether thefragments belonging to the combination are stored on servers or a P2Pdevice. In one embodiment, the fragments are distributed approximatelyat the same time, and a short while after being encoded, to both serversand P2P devices. The described system is similar to the hybridServers-P2P system with N unique erasure-coded fragments 550 to 559illustrated by FIG. 33, with the additional benefit of being able tosupport a broadcast effect.

In one embodiment, the first and the second portions comprise uniquerateless-coded fragments encoded with a redundancy factor that isdetermined according to the estimated popularity of the content beingencoded, whereby the higher the popularity of the content, the higherthe redundancy factor used to encode the content into fragments, and thelarger the number of bandwidth amplification devices that store thefragments associated with the content.

In one example, two contents are encoded into fragments usingrateless-coding. The first content is encoded with a redundancy factorgreater than 100, and the second content, which is estimated to be morepopular, with a redundancy factor greater than 1,000. Approximately allfragments are unique. The first content's fragments are distributed to100 fractional storage servers and to about 10,000 bandwidthamplification devices. The second content's fragments are distributed tothe same 100 fractional storage servers and to about 100,000 bandwidthamplification devices. Assuming that the fragment delivery bandwidths ofeach server and amplification device are 100 Mbps and 1 Mbpsrespectively, the first content has a potential delivery bandwidth of(100×100 Mbps)+(1,000×1 Mbps)=11 Gbps, and the second content has apotential delivery bandwidth of (100×100 Mbps)+(10,000×1 Mbps)=20 Gbps.If different popularities are expected, or alternatively differentpopularities are encountered during streaming of the contents, thendifferent redundancy factors may be used, and different numbers ofamplification devices per content may be utilized accordingly.

FIG. 35 illustrates one embodiment in which fractional-storage servers3799 a to 3799 c store a first portion of rateless-coded fragments; anda large number of P2P bandwidth amplification devices 3799 d to 3799 jstore a second portion of the rateless-coded fragments. Decodable setsof fragments can be obtained from combinations of fragments from thefirst and second portions of the fragments. Optionally, the fragmentsare obtained approximately only from P2P devices serviced by ISPs 3771,3772 having communication lines estimated not to be overloaded byadditional fragment traffic.

In one embodiment, the P2P devices are spread over different time zonesspanning at least three hours, and the fragments are obtainedapproximately only from P2P devices located in time zones in which thecurrent local Internet traffic is relatively low in comparison to peaklocal traffic.

In one example, ISP 3771 is located on the US west coast, and ISP 3772is located in Europe. At 6 PM PST, the general Internet trafficassociated with ISP 3771 is at its peak level, meaning thatcommunication line 3781 used by ISP 3771 is at or close to its maximaltraffic capacity. At that time, the local time in Europe is midnight,and the general Internet traffic associated with ISP 3772 is at a lowlevel, meaning that communication line 3782 used by ISP 3772 has asignificant unutilized bandwidth. In this case, fragments are obtainedby assembling devices approximately only from device combinationscomprising servers 3799 a to 3799 c, and amplification devices 3799 h to3799 j. Fragment delivered by devices 3799 h to 3799 j pass via theuncongested communication line 3782, and thus pose no threat to ISP3772. Communication line 3781 is not utilized for fragment delivery, andthus devices 3799 d to 3799 g do not contribute additional traffic loadsto the already congested line 3781. At a different time of day thesituation is reversed, the European ISP becomes congested while the USISP is at low capacity, and the fragments are obtained accordingly.

In one embodiment, all fractional-storage servers within the serverarray are replaced with client devices residing in customers' premises(CPEs). The CPEs perform the exact same functions as the server array,and in that respect may be referred to as a CPE array.

FIG. 36 illustrates one embodiment, wherein segment 101 a of content 100is encoded into erasure-coded fragments 390 a to 390(M), such that anysufficient subset of the fragments can be used to reconstruct segment101 a. Fragments 390 a to 390(N) are stored in fractional-storageservers 399 a to 399(N) respectively, and fragments 390(N+1) to 390(M)are stored in streaming server 399S. In one example, fragments 390(N+1)to 390(M) form a group of fragments which are sufficient to reconstructsegment 101 a. Subsequent segments 101 b to 101 j of content 100 may besimilarly encoded into additional fragments stored on the servers (notillustrated). Assembling device 309 uses two different protocolsapproximately simultaneously to retrieve fragments for segmentreconstruction: (i) a push protocol, and (ii) a fragment pull protocol.The push protocol 301S is used to deliver fragments 390(N+1) to 390(M)to assembling device 309. The push protocol may be RTP based orTCP-connection based, or any other type of transmission that does notrequire assembling device 309 to explicitly ask for each of fragments390(N+1) to 390(M). In one example, fragments 390(N+1) to 390(M) aredelivered to the assembling device using a single RTP stream 301S, suchthat upon reception of the fragments from the stream, the assemblingdevice can immediately reconstruct segment 101 a. The fragment pullprotocol is used by the assembling device to retrieve additionalfragments that may be needed to reconstruct segment 101 a if one or morefragments out of fragments 390(N+1) to 390(M) fail to reach theassembling device. In one example, fragment 390(N+2) fails to reach theassembling device due to Internet packet loss conditions (referred to asfragment loss). The assembling device, after concluding that fragment390(N+2) is missing, uses a fragment pull protocol to retrieve asubstitute fragment out of one of the fractional-storage servers 390 ato 390(N), and uses this fragment to complete the reconstruction of thesegment 101 a (any one of fragments 390 a to 390(N) will do). Forexample, the assembling device chooses fragment 390 a as the oneadditional fragment, by requesting and receiving it 303 a from server399 a, using a fragment pull protocol. If more fragments out offragments 390(N+1) to 390(M) fail to reach the assembling device 309, itmay compensate by pulling substitute fragments from some or all ofservers 399 a to 399(N), illustrated as fragment pull protocol requestsand responses 303 a to 303(N)).

In one embodiment, the fragment pull protocol requests for additionalneeded fragments are not made to fractional-storage servers 399 a to399(N), but are rather made to server 399S. In this case, the assemblingdevice asks server 399S to retransmit the fragment which has failed toarrive. In this embodiment, only fragments that fail to reach theassembling device via the push transmission 301S cause an addedcommunication overhead in the form of explicit fragment pull protocolrequests, such that if no fragments are actually lost over transmission301S, there is no need for fragment pull requests 303 a to 303(N).

In some embodiments, the push protocol is implemented using one or moresub-transmissions. Optionally, a push protocol transmission isimplemented using multiple sub-transmissions, each transporting afraction of the fragments transmitted by the push protocol transmission.A sub-transmission may be transported using an IP stream such as RTP, anHTTPS session, or any other form of transporting a sequence of fragmentsbetween a source server and a destination assembling device.

In one embodiment, an assembling device starts retrieving fragmentsusing only fragment pull protocol processes, and then, when concludingthat a specific server is responsive enough, instructs it to startsending a push-transmission for the remaining segments. In this case,the assembling device may start with pure pull-protocol based fragmentretrieval, and gradually switch to push-protocol transmissions, up tothe point that approximately all fragments are delivered usingpush-transmissions, and using the pull requests only as a means toovercome failure of obtaining specific fragments by the assemblingdevice. In one embodiment, the fragment pull protocol and the pushprotocol are used interchangeably to obtain enough fragments toreconstruct segments. In one embodiment, the assembling device may startto obtain fragments using a push protocol and then switch to a fragmentpull protocol. In one embodiment, the assembling device may use bothfragment pull protocol and push protocol to obtain fragments at the sametime, wherein the assembling device may change the ratio Fpull/Fpushon-the-fly to any value between zero and infinity, where Fpull denotesthe number of fragments associated with a certain segment that areobtained using a fragment pull protocol, and Fpush denotes the number offragments associated with the certain segment that are obtained using apush protocol.

In the claims, sentences such as “wherein the assembling device isconfigured to use a fragment pull protocol to obtain the fragments” and“wherein the assembling device is configured to use sub-transmissions toobtain the fragments” are to be interpreted as open claim language.Therefore, an assembling device configured to use a fragment pullprotocol to obtain fragments may also obtain fragments usingsub-transmissions, and vice-versa.

Referring again to FIG. 10 with device 6610 as a non-assembling CPE,such as a STB, PC or gaming console, capable of performing standardrequest, reception, and decoding of video over IP network. In oneembodiment, server 661 s—also referred to as proxy server, assemblingserver, and in some cases assembling device—performs three primaryfunctions: (i) receipt of content requests from non-assembling clientdevice 661 o; (ii) assembly of content, as requested by client 661 o,from the fractional-storage servers and optionally from the bandwidthamplification devices; (iii) optionally, conversion of the assembledcontent into a streaming format; and (iv) transmission of the streamingcontent to the requesting client 661 o. Client 6610 can then store thecontent, or present it. In one embodiment, the assembled content is ageneral web content, including HTML, FLASH or any other data format thatcan be found in a web-based site.

In one embodiment, although server 661 s is illustrated as beingconnected to network 300 on one side and to network 300 n on the other,server 661 s may also be connected to another network element, such as arouter, which makes the topological connection between networks 300 and300 n. In that case, server 661 s communicates with both networks 300and 300 n via the other network element.

In one embodiment, a CDN is created by the aggregated bandwidth andstorage capacity of the participating erasure-coded fractional-storageservers. In one example, a large scale CDN includes several hundreds orthousands of fractional-storage servers connected to the Internet. Theseservers send erasure-coded fragments to a large number, potentiallymillions, of assembling devices. In order to keep costs low for sendinga large number of fragments from fractional-storage servers toassembling devices, the servers are located on the Internet backbone, orclose to it.

The current Internet backbone primarily comprises different Tier one ISP(or other) networks that interconnect at various Internet ExchangePoints (IX or IXP), using peering agreements. Tier one ISPs, or otherbackbone-forming network entities, can reach any portion of the Internetvia other Tier one ISPs or other backbone-forming networks, withoutpaying any Internet transit fee, and solely by utilizing mutual peeringagreements. In order to gain access to large amounts of inexpensivebandwidth, the fractional-storage servers are typically located on theInternet backbone. This means that the servers are either co-located(and connected) with a core switching router that interconnects theInternet backbone networks at an IXP, or, alternatively, co-located (andconnected) with a router which is part of the backbone network,typically located at a data center or co-location center.Fractional-storage servers can also be located close to the Internetbackbone, which means that they are co-located (and connected) with arouter which is part of a Tier two ISP network, which has a highbandwidth connection with at least one Tier one operator, to which itpays transit fees in order to potentially reach all portions of theInternet. FIG. 37 illustrates one example of a fractional-storage server3001, which is one of a plurality of servers forming a large-scale CDN,located on the Internet backbone by being connected to the Internetbackbone via IXP 3091. In a second example, fractional-storage server3002 is located on the Internet backbone by being connected to a Tierone backbone network 3080. In a third example, fractional-storage server3011 is located close to the Internet backbone by being connected to aTier two ISP network 3070, which is connected to the backbone via Tierone ISP network 3081. In one embodiment, a typical fractional-storageserver is located on the backbone or close to the backbone by beingattached to a switching router via a high bandwidth port, such as a 1Gbps, 10 Gbps, or a higher bandwidth port, such as high-speed Ethernetport, usually carried over a fiber, or suitable short-distance copperlines. In one embodiment, in a typical deployment using high bandwidthconnections (in 2009 terms), each of about 1,000 fractional-storageservers is located on the backbone or close to the backbone and isconnected to the backbone via a dedicated (guaranteed bandwidth) 1 GbpsEthernet port, resulting in an aggregated throughput of 1,000 Gbps,which can serve about one million subscribers of standard definitionstreaming video, such as client device 3020, simultaneously. Suchaggregated bandwidths would have required a substantially larger numberof fractional-storage servers, had they been connected to otherlocations in the Internet, such as at edges of the Internet (close tolast mile networks), Tier 3 ISPs, or at the user premises. Moreover, insome embodiments, the cost of streaming the mentioned 1,000 Gbps whenthe fractional-storage servers are located on the Internet backbone, orclose to the Internet backbone, is expected to be significantly lowerthan what is expected when the servers are located elsewhere asmentioned before.

FIG. 38 illustrates one example where an assembling server 4020 islocated at the juncture 4010 between two networks: the first network isan ISP transit network 4014 that connects the juncture to the Internetand provides Internet transit via a switching router 4015, and thesecond is a last mile network 4041 that connects end users 4051 to theInternet via a switch 4031 (located, for example, inside a CentralOffice, a Head-End, or a street-level cabinet). In one embodiment, thejuncture 4010 is a network operated by a local ISP that pays transitfees for Internet traffic passing through the transit network 4014, andlast mile fees for traffic passing through the last mile network 4041. Aunique property of the juncture 4010 is that it is possible for anassembling server 4020 located at the juncture to receive erasure-codedfragments sent by fractional-storage servers, such as 4001 and 4002, toassemble content, and to stream the content to a client 4051 via thelast mile network 4041, without incurring any additional costs incomparison to other scenarios, such as where Internet packets flow fromthe Internet backbone to a Tier two ISP network to the Internet backboneand to the last mile network. In other words, since the assemblingserver 4020 is located at the juncture, it does not create any extratraffic via networks 4014 and 4041. The assembling server can also belocated at or close to an edge of the Internet, which may include thejuncture, or a point above server 4015, such as at the transit network4014 connecting the juncture to the Internet. When located at or closeto an edge of the Internet, the assembling server has the potential notto incur additional transit fees as a result of the relaying operation,since approximately the same traffic would have to pass via the sametransit network in a normal scenario. Another beneficial location forthe assembling server is at the home premises, since, clearly, arelaying operation performed there does not add any significant trafficto higher levels of the network. In contrast to the above-suggestedlocations, in some cases an assembling server may be located at anarbitrary point on the backbone, or at other high-level points of theInternet, where it incurs additional transit fees, as fragmentsassembled by the server flow once over an Internet transit network goingfrom a fractional-storage server to the assembling server, and then asecond time when streamed by the assembling server to a destinationclient over an Internet transit network.

In one embodiment, different groups of fractional-storage servers, alsoreferred to as data centers, are owned by different owners and/orconnected to the Internet by different entities, and the assemblingdevices balance the load on the fractional-storage servers.

In one embodiment, different data centers host fractional-storageservers. The servers store erasure-coded fragments encoded with aredundancy factor R greater than one. A plurality of assembling devicesobtain fragments needed for streaming contents. No group of serverswithin any one of the data centers store more than (1−1/R) of thefragments associated with a single segment to be reconstructed; meaningthat if any one of the data centers stop delivering fragments, the otherdata centers still comprise enough erasure-coded fragments needed todecode the fragments. Upon termination of a fragment delivery servicefrom any one of the data centers, the servers of the data center whosefragment delivery service was terminated are deselected for fragmentdelivery, and other servers in other data centers are selected forfragment delivery instead, while the streaming of the contents toaffected assembling devices continues during this shortdeselection-reselection process, and without disrupting any ongoingstreaming operation. Usually, each data center hosts more than oneserver, but a data center may also host a single server. In oneembodiment, more than 200 servers are hosted in more than 20 datacenters. In one embodiment, more than 10,000 servers are hosted in morethan 100 data centers, and deliver fragments at an aggregated throughputof more than 10 Tera-bit per second.

In one embodiment, different data centers feature significantlydifferent characteristics. Examples of different characteristics include(i) available storage space, (ii) available fragment deliverythroughput, (iii) latency in response to fragment requests, (iv) adifference in the tier or size of the network to which the data centeris directly connected, (v) ownership of the data center, (vi) operationcost, (vii) data center outage periods and frequency, (viii) data centerreliability, and/or (ix) the nature or capacity of the line connectingthe data center to the Internet.

In one embodiment, some of the data centers may belong to one or more ofthe following entities: a hosting provider, a backbone or Tier-1 networkoperator, and/or a corporation.

In one embodiment, the data center is any structure connected to theInternet, and providing an Internet connection to at least one housedfractional-storage server. In one embodiment, the data center isconnected with a fixed bandwidth link to the Internet. In oneembodiment, the data center is connected to a router that is a part ofan Internet backbone or Tier-1 network.

In one embodiment, the assembling devices 1499 use a fragment pullprotocol to retrieve fragments from the servers and to approach newservers, optionally on-the-fly while streaming contents, instead ofservers whose data center's fragment-delivery service was terminated.

In one embodiment, the assembling devices 1499 use a push protocol toobtain fragments from the servers by utilizing multiplesub-transmissions. Servers whose data center's fragment-delivery servicewas terminated are replaced, optionally on-the-fly while streamingcontents, by other servers which operate as the sub-transmissionsources.

In one example, the deselection-reselection process takes a few secondsor less. In one example, the deselection-reselection process is donefaster than it takes to play the content stored in one relatively shortsegment. In one example, the deselection-reselection process is done byassembling devices. In one embodiment, the reselection of servers isdone by a control server.

In one embodiment, termination of fragment delivery service from any oneof the data centers may be a result of the data center underperformingin comparison to other data centers. The termination in this case isusually triggered by the fragment-delivery service operator. Examples ofunderperformance include (i) the cost of delivering data being higherthan delivering data from other centers, (ii) the transmitted fragmentsbeing subject to higher fragment loss rate as compared to other centers,(iii) the transmitted fragments being subject to higher latency orlatency variance as compared to other centers, and/or (iv) the outageperiods being longer or more frequent than those of other centers.

In one embodiment, termination of fragment delivery service from any oneof the data centers may be a result of the data center operator orhosting provider not wanting to continue hosting the fractional-storageservers. Such termination may be abrupt and without a warning.

In one embodiment, just before service termination, the aggregatedunutilized fragment-delivery bandwidth available to the alternativeservers is larger than the fragment delivery throughput via the datacenter whose service is terminated. In other words, there is enoughbandwidth among the remaining servers to support the streamingthroughput of the system prior to the termination event.

In one embodiment, just before service termination, the terminatedservice is still capable of delivering a substantial fragment throughputbut is underperforming in comparison to the other data centers. In otherwords, the center is still able to perform prior to service termination.

In one embodiment, servers housed in data centers whose fragmentdelivery service is about to be terminated, are excluded from the poolof servers considered by assembling devices or control servers as validsources of fragments. The service is then terminated after approximatelyall assembling devices have stopped using these servers as fragmentsources.

In one embodiment, a new data center housing servers storing uniquefragments is added to the service before the step of terminating thefragment delivery service of a certain center. In this case, theaggregated unutilized fragment delivery bandwidth of the new data centerand the remaining data centers is larger than the fragment deliverythroughput supplied by the service to be terminated.

In one embodiment, from time to time and on a regular basis, the systemadds and excludes data centers from the streaming operations, whilemaintaining continuous streaming. In one embodiment, data centers thatunderperform compared to the other data centers are candidates forexclusion. In one embodiment, most of the excluded data centers providedfragments to approximately the smallest number of assembling devicesover a predefined period before their exclusion.

In one embodiment, the erasure-coding used is a rateless-coding,enabling practically an infinite number of unique fragments. Therefore,it is possible to assign unique fragments to servers of newly added datacenters, regardless of how many data centers are added and removed, andat what frequency. New and unique fragments can always be calculated andused. In one embodiment, the fragments stored on servers of the excludeddata center are usually not stored again on servers of the remaining orto be added data centers, especially when using rateless coding.

In one embodiment, over a long period, there are more additions of datacenters than exclusions, and therefore the streaming and storagecapacity of the streaming system increases.

In the claims, a sentence such as “erasure-coded fragments encoded witha redundancy factor R>1 and associated with segments of streamingcontents” is to be interpreted as erasure-coded fragments encoded withone redundancy factor or with a plurality of redundancy factors greaterthan one. For example, some fragments associated with a first set ofsegments of content may have a redundancy factor of two, and somefragments associated with a second set of segments of the same contentmay have a redundancy factor of three.

In one embodiment, fractional-storage servers within data centers storeerasure-coded fragments encoded with a redundancy factor greater thanone. A plurality of assembling devices obtain decodable sets offragments from subsets of the servers and measure fragment deliveryparameters that are indicative of delivery performances, such as latencyin responding to requests, or fragment loss ratios. Each assemblingdevice can readily make the measurements on fragments sent to it.Decisions are constantly made by the assembling devices, a controlserver, or any other decision component, regarding selection andreselection of servers participating in the subsets. The decisions arebased on the measured parameters, and are made in order to improve themeasured parameters. After many such decisions are made for or by manyassembling devices, it is possible to estimate the performances of thedifferent data centers. A data center that is underperforming relativeto other data centers is likely to feature one or more of the following:(i) delivers fewer fragments to assembling devices as compared to otherdata centers, (ii) incurs higher cost per fragment delivery, and thus isless cost effective compared to other data centers, (iii) utilizes alower percentage of the fragment delivery bandwidth available to it, ascompared to other centers, and/or (iv) exhibits any other measurabledegradation in performance level, that is a result of serverparticipation in subsets, and that can be used to differentiate it overwell performing data centers. The preference of the assembling devices,or other decision component, for some servers over other servers createsa “natural selection” process that can be utilized to distinguish wellperforming data centers over underperforming data centers. After thedata centers are distinguished, decisions can be made regarding a futureutilization of each center.

In one embodiment, one or more of the following is used as the measuredfragment delivery parameters: latency in responding to data requests,variance in latency in responding to fragment requests, fragment loss,service outage, and/or reported load level encountered by the serverswhen delivering fragments. In one embodiment, the assembling devices areconfigured to obtain the fragments using a fragment pull protocol thatis used for estimating at least one of the parameters.

An underperforming data center is likely to include servers that areless frequently selected for participation in subsets than serversbelonging to other well performing data centers. This, in turn, reducesthe fragment delivery throughput from the underperforming center, ascompared to other centers. In one embodiment, centers having lowerdelivery throughputs over time are excluded from the system.

In one embodiment, an underperforming data center has a higher cost ofdelivering a fragment than other centers. A center that includes serversthat are less frequently selected for participation in subsets, comparedto servers belonging to other well performing data centers, will havelower fragment delivery throughput as compared to other centers.Assuming that the fragment delivery operator is paying a fixed price forthat center's delivery services, the result is a decrease in fragmentdelivery cost efficiency. In this case, one option is to exclude theunderperforming center from the system, and to stop delivering fragmentsfrom it. Another option is to reduce the amount of bandwidth acquiredfrom that center, to a level that is more appropriate to actualthroughputs, or to downscale the service agreement and reduce the fixedprice. This may increase the cost efficiency of the center back to anacceptable level. If further reduction in throughputs are observed, andcost efficiency falls again, then the process of reducing the acquiredbandwidth or downscaling the service agreement can be repeated, untilpossibly eliminating the center as a fragment source.

In one embodiment, a distributed system is located in a few to dozens ofdata centers (also known as server farm or datacenter), located close toor on the Internet backbone, together housing at least 100fractional-storage CDN servers. The servers store erasure-codedfragments associated with approximately sequential segments of streamingcontents, with a storage gain of at least 5, and transmit the storedfragments on demand to assembling devices approximately according to thesequential order of the segments. In many cases, the data centersprovide a convenient place to place the CDN servers close to or on theInternet backbone. A data center can be also a collocation center, or anInternet Exchange Point. In one example, a single data center can housemany fractional-storage CDN servers.

In one example, a streaming system comprising at least several hundredsof fractional-storage CDN servers located close to or on the Internetbackbone, storing erasure-coded fragments encoded with a redundancyfactor greater than one, and associated with approximately sequentialsegments of streaming contents. At least 100,000 assembling devicesconcurrently obtain fragments from the CDN servers, wherein the systemachieves efficient load balancing and fault tolerance between thevarious CDN servers by determining for each of the assembling devicesfrom which servers to obtain the fragments.

In one example, a system comprising at least 1,000 fractional-storageCDN servers is connected to the public Internet. The servers storeerasure-coded fragments associated with approximately sequentialsegments of streaming contents, with a storage gain greater than 5, andtransmit the stored fragments on demand to assembling devicesapproximately according to the sequential order of the segments. Whereinthe aggregated bandwidth utilized by the servers for transmitting thefragments to the assembling devices exceeds 1 Giga bit per second timesthe number of the CDN servers. In one optional example, the systemcomprises at least 10,000 fractional-storage CDN servers and theaggregated bandwidth utilized by the servers exceeds 10 Giga bit persecond times the number of the CDN servers.

In the claims, a sentence such as “the erasure-coded fragments supportsource-selection diversity” is to be interpreted as fragments encodedusing any kind of erasure-code that can produce N unique fragments, fromwhich C combinations of decodable sets of fragments can be selected,wherein C is much greater than N. Standard parity checks, standardchecksums, and standard cyclic redundancy checks (CRC) are examples ofcodes that do not support source-selection diversity.

In this description, numerous specific details are set forth. However,the embodiments of the invention may be practiced without some of thesespecific details. In other instances, well-known hardware, software,materials, structures and techniques have not been shown in detail inorder not to obscure the understanding of this description. In thisdescription, references to “one embodiment” mean that the feature beingreferred to may be included in at least one embodiment of the invention.Moreover, separate references to “one embodiment” or “some embodiments”in this description do not necessarily refer to the same embodiment.Illustrated embodiments are not mutually exclusive, unless so stated andexcept as will be readily apparent to those of ordinary skill in theart. Thus, the invention may include any variety of combinations and/orintegrations of the features of the embodiments described herein.

Although some embodiments may depict serial operations, the embodimentsmay perform certain operations in parallel and/or in different ordersfrom those depicted. Moreover, the use of repeated reference numeralsand/or letters in the text and/or drawings is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed. Theembodiments are not limited in their applications to the details of theorder or sequence of steps of operation of methods, or to details ofimplementation of devices, set in the description, drawings, orexamples. Moreover, individual blocks illustrated in the figures may befunctional in nature and do not necessarily correspond to discretehardware elements. While the methods disclosed herein have beendescribed and shown with reference to particular steps performed in aparticular order, it is understood that these steps may be combined,sub-divided, or reordered to form an equivalent method without departingfrom the teachings of the embodiments. Accordingly, unless specificallyindicated herein, the order and grouping of the steps is not alimitation of the embodiments. Furthermore, methods and mechanisms ofthe embodiments will sometimes be described in singular form forclarity. However, some embodiments may include multiple iterations of amethod or multiple instantiations of a mechanism unless noted otherwise.For example, when a controller or an interface are disclosed in anembodiment, the scope of the embodiment is intended to also cover theuse of multiple controllers or interfaces.

Certain features of the embodiments, which may have been, for clarity,described in the context of separate embodiments, may also be providedin various combinations in a single embodiment. Conversely, variousfeatures of the embodiments, which may have been, for brevity, describedin the context of a single embodiment, may also be provided separatelyor in any suitable sub-combination.

Embodiments described in conjunction with specific examples arepresented by way of example, and not limitation. Moreover, it is evidentthat many alternatives, modifications and variations will be apparent tothose skilled in the art. It is to be understood that other embodimentsmay be utilized and structural changes may be made without departingfrom the scope of the embodiments. Accordingly, it is intended toembrace all such alternatives, modifications and variations that fallwithin the spirit and scope of the appended claims and theirequivalents.

1) A method comprising: for each group of at least one assemblingdevice, selecting a subgroup of fractional-storage CDN servers accordingto at least one criterion, whereby a plurality of server subgroups areselected for a plurality of assembling device groups; and retrievingusing a fragment pull protocol, by the assembling devices from thesubgroups of servers, erasure-coded fragments associated with segmentsof contents, until the aggregated bandwidth used for retrieving thefragments approaches the aggregated bandwidth of the servers included inthe subgroups, and as long as the aggregated bandwidth used fordelivering each segment does not exceed the aggregated bandwidth of theservers storing the fragments encoded from the segment. 2) The method ofclaim 1, further comprising repeating the step of selecting the subgroupof servers occasionally on-the-fly. 3) The method of claim 2, whereinthe fragment pull protocol is a fragment pull protocol for high latency,each server stores at least one fragment per approximately each segmentof each content, and whereby it is possible for any content to beretrieved utilizing a bandwidth approaching the aggregated bandwidth ofthe servers. 4) The method of claim 3, further comprising the step ofdistributing the fragments to the servers with approximately no priorknowledge of the future bandwidth demand for retrieving each content. 5)The method of claim 2, wherein the reselection is performed by a controlserver, whereby the decisions regarding the selected servers aretransmitted to the relevant assembling devices. 6) The method of claim2, wherein selecting the subgroup of servers according to at least onecriterion comprises selecting the servers having lower loads and lowerlatencies than most of their alternatives, and the fragment pullprotocol is a fragment pull protocol for high latency. 7) The method ofclaim 2, wherein the reselection is performed by each assembling devicefor itself, and the erasure-coding is rateless-coding potentiallyresulting in fragments having a limitless redundancy factor. 8) Themethod of claim 2, wherein the reselection is performed by eachassembling device, the reselection is performed approximately per eachsegment or group of segments, and approximately all of the assemblingdevices use the same criterion for selecting the subgroups. 9) Themethod of claim 2, wherein a first portion of the assembling devices usea first criterion and a second portion of the assembling devices use asecond criterion for selecting the servers in the subgroups. 10) Themethod of claim 2, further comprising reselecting the subgroups whileretrieving the fragments associated with the beginnings of the contents.11) The method of claim 1, further comprising repeating the step ofselecting the servers occasionally on-the-fly for only some of theservers. 12) A method comprising: obtaining by assembling devices, fromsubgroups of fractional-storage servers, erasure-coded fragments encodedfrom segments of at least one content; and changing at least one of theservers in the subgroups on-the-fly from time to time utilizing afragment pull protocol; wherein different mixtures of segments can besimultaneously obtained by approximately any number of assemblingdevices, until the aggregated bandwidth used for obtaining the segmentsapproaches the available aggregated bandwidth of the servers, and aslong as the aggregated bandwidth used to deliver each segment does notexceed the aggregated bandwidth of the servers storing the fragmentsassociated with the segment. 13) The method of claim 12, wherein eachserver stores at least one fragment per approximately each segment ofeach content, and the assembling devices are using a fragment pullprotocol for obtaining at least most of the fragments from the servers.14) The method of claim 12, wherein the assembling devices are using afragment pull protocol for obtaining at least most of the fragments fromthe servers, and the step of changing at least one of the servers in thesubgroups comprises reselecting the servers approximately randomly fromall of the available servers; and wherein the erasure-coding israteless-coding potentially resulting in fragments having a limitlessredundancy factor. 15) The method of claim 12, wherein the assemblingdevices are using a fragment pull protocol for obtaining the fragmentsfrom the servers, and the step of changing at least one of the serversin the subgroups comprises changing approximately randomly only aportion of the servers in the subgroups. 16) The method of claim 12,wherein the erasure-coding is rateless-coding, the assembling devicesare using a fragment pull protocol for obtaining the fragments from theservers, and further comprising the step of distributing the fragmentsto the servers with approximately no prior knowledge of the futurebandwidth demand for obtaining each segment. 17) The method of claim 12,wherein the servers are fractional-storage CDN servers located close toor on the Internet backbone, and the assembling devices obtaining thefragments via at least two sub-transmissions transmitted by the servers.18) A distributed system comprising: CDN servers configured to storeenough erasure-coded fragments to reconstruct segments of streamingcontent; for each segment, approximately each CDN server is configuredto store less than the minimal amount of erasure-coded fragments neededto reconstruct the segment; the distributed system is further configuredto provide erasure-coded fragments to assembling devices at a totalthroughput approaching its maximum aggregated bandwidth, while supplyingerasure-coded fragments to reconstruct approximately any possiblemixture of streaming contents; wherein one or more of the assemblingdevices can replace one or more of the servers they retrieve from, withdifferent servers, approximately instantaneously. 19) The distributedsystem of claim 18, wherein the assembling devices are configured to usea fragment pull protocol to retrieve at least most of the fragments fromthe CDN servers. 20) The distributed system of claim 18, wherein the CDNservers are located close to or on the Internet backbone, at least mostof the fragments are supplied via a plurality of sub-transmissions, andthe erasure-coding is rateless-coding.